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-color-config/src/nu_style.rs

@@ -65,7 +65,7 @@ fn color_to_string(color: Color) -> Option<String> {
         Color::White => Some(String::from("white")),
         Color::LightGray => Some(String::from("light_gray")),
         Color::Default => Some(String::from("default")),
-        Color::Rgb(r, g, b) => Some(format!("#{:X}{:X}{:X}", r, g, b)),
+        Color::Rgb(r, g, b) => Some(format!("#{r:X}{g:X}{b:X}")),
         Color::Fixed(_) => None,
     }
 }