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/core_commands/ast.rs

@@ -41,7 +41,7 @@ impl Command for Ast {
         let mut working_set = StateWorkingSet::new(engine_state);
 
         let (output, err) = parse(&mut working_set, None, pipeline.item.as_bytes(), false, &[]);
-        eprintln!("output: {:#?}\nerror: {:#?}", output, err);
+        eprintln!("output: {output:#?}\nerror: {err:#?}");
 
         Ok(PipelineData::empty())
     }

crates/nu-command/src/core_commands/help.rs

@@ -195,7 +195,7 @@ pub fn highlight_search_string(
     needle: &str,
     string_style: &Style,
 ) -> Result<String, ShellError> {
-    let regex_string = format!("(?i){}", needle);
+    let regex_string = format!("(?i){needle}");
     let regex = match Regex::new(&regex_string) {
         Ok(regex) => regex,
         Err(err) => {