Use variable names directly in the format strings (#7906)

# Description

Lint: `clippy::uninlined_format_args`

More readable in most situations.
(May be slightly confusing for modifier format strings
https://doc.rust-lang.org/std/fmt/index.html#formatting-parameters)

Alternative to #7865

# User-Facing Changes

None intended

# Tests + Formatting

(Ran `cargo +stable clippy --fix --workspace -- -A clippy::all -D
clippy::uninlined_format_args` to achieve this. Depends on Rust `1.67`)

crates/nu-command/tests/commands/platform/kill.rs

@@ -3,7 +3,7 @@ use nu_test_support::nu;
 #[test]
 fn test_kill_invalid_pid() {
     let pid = i32::MAX;
-    let actual = nu!(format!("kill {}", pid));
+    let actual = nu!(format!("kill {pid}"));
 
     assert!(actual.err.contains("process didn't terminate successfully"));
 }