Fix clippy::uninlined_format_args warning

```
error: variables can be used directly in the `format!` string
  --> examples/timeout.rs:75:9
   |
75 |         println!("Stdout:\n{}", stdout);
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#uninlined_format_args
   = note: `-D clippy::uninlined-format-args` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(clippy::uninlined_format_args)]`
help: change this to
   |
75 -         println!("Stdout:\n{}", stdout);
75 +         println!("Stdout:\n{stdout}");
   |

error: variables can be used directly in the `format!` string
  --> examples/timeout.rs:76:9
   |
76 |         println!("Stderr:\n{}", stderr);
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#uninlined_format_args
help: change this to
   |
76 -         println!("Stderr:\n{}", stderr);
76 +         println!("Stderr:\n{stderr}");
   |

error: could not compile `async-process` (example "timeout") due to 2 previous errors
warning: build failed, waiting for other jobs to finish...
error: variables can be used directly in the `format!` string
  --> tests/std.rs:80:23
   |
80 |             result => panic!("not terminated by signal 9 (instead, {:?})", result),
   |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#uninlined_format_args
   = note: `-D clippy::uninlined-format-args` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(clippy::uninlined_format_args)]`
help: change this to
   |
80 -             result => panic!("not terminated by signal 9 (instead, {:?})", result),
80 +             result => panic!("not terminated by signal 9 (instead, {result:?})"),
   |

error: variables can be used directly in the `format!` string
   --> tests/std.rs:335:9
    |
335 | /         assert!(
336 | |             output.contains("RUN_TEST_NEW_ENV=123"),
337 | |             "didn't find RUN_TEST_NEW_ENV inside of:\n\n{}",
338 | |             output
339 | |         );
    | |_________^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#uninlined_format_args

error: variables can be used directly in the `format!` string
   --> tests/std.rs:353:9
    |
353 | /         assert!(
354 | |             output.contains("RUN_TEST_NEW_ENV=123"),
355 | |             "didn't find RUN_TEST_NEW_ENV inside of:\n\n{}",
356 | |             output
357 | |         );
    | |_________^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#uninlined_format_args

error: variables can be used directly in the `format!` string
   --> tests/std.rs:375:9
    |
375 | /         assert!(
376 | |             output.contains("RUN_TEST_NEW_ENV1=123"),
377 | |             "didn't find RUN_TEST_NEW_ENV1 inside of:\n\n{}",
378 | |             output
379 | |         );
    | |_________^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#uninlined_format_args

error: variables can be used directly in the `format!` string
   --> tests/std.rs:380:9
    |
380 | /         assert!(
381 | |             output.contains("RUN_TEST_NEW_ENV2=456"),
382 | |             "didn't find RUN_TEST_NEW_ENV2 inside of:\n\n{}",
383 | |             output
384 | |         );
    | |_________^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#uninlined_format_args
```

Author: taiki-e

examples/timeout.rs

@@ -72,8 +72,8 @@ fn main() -> io::Result<()> {
             println!("The child exited.");
         }
 
-        println!("Stdout:\n{}", stdout);
-        println!("Stderr:\n{}", stderr);
+        println!("Stdout:\n{stdout}");
+        println!("Stderr:\n{stderr}");
 
         Ok(())
     })

tests/std.rs

@@ -77,7 +77,7 @@ fn signal_reported_right() {
         p.kill().unwrap();
         match p.status().await.unwrap().signal() {
             Some(9) => {}
-            result => panic!("not terminated by signal 9 (instead, {:?})", result),
+            result => panic!("not terminated by signal 9 (instead, {result:?})"),
         }
     })
 }
@@ -334,8 +334,7 @@ fn test_override_env() {
 
         assert!(
             output.contains("RUN_TEST_NEW_ENV=123"),
-            "didn't find RUN_TEST_NEW_ENV inside of:\n\n{}",
-            output
+            "didn't find RUN_TEST_NEW_ENV inside of:\n\n{output}"
         );
     })
 }
@@ -352,8 +351,7 @@ fn test_add_to_env() {
 
         assert!(
             output.contains("RUN_TEST_NEW_ENV=123"),
-            "didn't find RUN_TEST_NEW_ENV inside of:\n\n{}",
-            output
+            "didn't find RUN_TEST_NEW_ENV inside of:\n\n{output}"
         );
     })
 }
@@ -374,13 +372,11 @@ fn test_capture_env_at_spawn() {
 
         assert!(
             output.contains("RUN_TEST_NEW_ENV1=123"),
-            "didn't find RUN_TEST_NEW_ENV1 inside of:\n\n{}",
-            output
+            "didn't find RUN_TEST_NEW_ENV1 inside of:\n\n{output}"
         );
         assert!(
             output.contains("RUN_TEST_NEW_ENV2=456"),
-            "didn't find RUN_TEST_NEW_ENV2 inside of:\n\n{}",
-            output
+            "didn't find RUN_TEST_NEW_ENV2 inside of:\n\n{output}"
         );
     })
 }