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/tests/commands/save.rs

@@ -16,7 +16,7 @@ fn writes_out_csv() {
         );
 
         let actual = file_contents(expected_file);
-        println!("{}", actual);
+        println!("{actual}");
         assert!(actual.contains("nu,0.14,A new type of shell,MIT,2018"));
     })
 }
@@ -52,7 +52,7 @@ fn save_append_will_create_file_if_not_exists() {
         );
 
         let actual = file_contents(expected_file);
-        println!("{}", actual);
+        println!("{actual}");
         assert_eq!(actual, "hello");
     })
 }
@@ -78,7 +78,7 @@ fn save_append_will_not_overwrite_content() {
         );
 
         let actual = file_contents(expected_file);
-        println!("{}", actual);
+        println!("{actual}");
         assert_eq!(actual, "hello world");
     })
 }