chore: fix cargo clippy warning

[xixishidibei]

Running cargo clippy reports the following error.

warning: variables can be used directly in the `format!` string
  --> src/assert.rs:64:17
   |
64 |                 panic!("Failed to spawn {:?}: {}", self, err);
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#uninlined_format_args
   = note: requested on the command line with `-W clippy::uninlined-format-args`
help: change this to
   |
64 -                 panic!("Failed to spawn {:?}: {}", self, err);
64 +                 panic!("Failed to spawn {self:?}: {err}");
   |

warning: `assert_cmd` (lib) generated 1 warning (run `cargo clippy --fix --lib -p assert_cmd` to apply 1 suggestion)

This commit resolves the issue.

[coveralls]

Pull Request Test Coverage Report for Build 14395017533

Details

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 44.856%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/assert.rs	0	1	0.0%

Totals
Change from base Build 14241399667:	0.0%
Covered Lines:	218
Relevant Lines:	486

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💛 - Coveralls