ci: Disable building std for AIX

```
  error[E0425]: cannot find function `getenv` in this scope
     --> /home/runner/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sys/pal/unix/os.rs:285:22
      |
  285 |     if let Some(p) = getenv(OsStr::from_bytes("PATH".as_bytes())) {
      |                      ^^^^^^ not found in this scope
      |
  help: consider importing one of these functions
      |
  8   + use crate::sys::env::getenv;
      |
  8   + use libc::getenv;
      |
```

Author: taiki-e

Cargo.toml

@@ -84,6 +84,8 @@ critical-section = { version = "1", optional = true }
 
 # [target.'cfg(portable_atomic_test_no_std_static_assert_ffi)'.dependencies] #build:static_assert_ffi
 # test-helper = { features = ["sys"], git = "https://github.com/taiki-e/test-helper.git", rev = "93816e3" } #build:static_assert_ffi
+# [target.'cfg(all(portable_atomic_test_no_std_static_assert_ffi, target_os = "aix"))'.dependencies] #build:static_assert_ffi
+# libc = "=0.2.163" #build:static_assert_ffi
 
 [dev-dependencies]
 build-context = "0.1"

tools/build.sh

@@ -159,9 +159,9 @@ is_no_std() {
   case "$1" in
     *-linux-none*) ;;
     # https://github.com/rust-lang/rust/blob/1.84.0/library/std/build.rs#L65
-    # ESP-IDF supports std, but it is often broken.
+    # ESP-IDF and AIX supports std, but they are often broken.
     # aarch64-unknown-linux-uclibc is a custom target and libc/std currently doesn't support it.
-    *-none* | *-psp* | *-psx* | *-cuda* | avr* | *-espidf | aarch64-unknown-linux-uclibc) return 0 ;;
+    *-none* | *-psp* | *-psx* | *-cuda* | avr* | *-espidf | *-aix | aarch64-unknown-linux-uclibc) return 0 ;;
   esac
   return 1
 }