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-cli/src/prompt.rs

@@ -91,7 +91,7 @@ impl NushellPrompt {
     }
 
     fn default_wrapped_custom_string(&self, str: String) -> String {
-        format!("({})", str)
+        format!("({str})")
     }
 }