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_plugin_inc/src/inc.rs

@@ -121,7 +121,7 @@ impl Inc {
             x => {
                 let msg = x.as_string().map_err(|e| LabeledError {
                     label: "Unable to extract string".into(),
-                    msg: format!("value cannot be converted to string {:?} - {}", x, e),
+                    msg: format!("value cannot be converted to string {x:?} - {e}"),
                     span: Some(head),
                 })?;