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-engine/src/eval.rs

@@ -19,7 +19,7 @@ pub fn eval_operator(op: &Expression) -> Result<Operator, ShellError> {
             ..
         } => Ok(operator.clone()),
         Expression { span, expr, .. } => {
-            Err(ShellError::UnknownOperator(format!("{:?}", expr), *span))
+            Err(ShellError::UnknownOperator(format!("{expr:?}"), *span))
         }
     }
 }