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/src/system/run_external.rs

@@ -753,7 +753,7 @@ fn shell_arg_escape(arg: &str) -> String {
         s if !has_unsafe_shell_characters(s) => String::from(s),
         _ => {
             let single_quotes_escaped = arg.split('\'').join("'\"'\"'");
-            format!("'{}'", single_quotes_escaped)
+            format!("'{single_quotes_escaped}'")
         }
     }
 }