Fix easy clippy lints from latest stable (#16053)

1.88.0 was released today, clippy now lints (machine-applicable)
against:
- format strings with empty braces that could be inlined
  - easy win
- `manual_abs_diff`
- returning of a stored result of the last expression.
  - this can be somewhat contentious but touched only a few places

crates/nu-command/tests/format_conversions/msgpack.rs

@@ -8,7 +8,7 @@ fn msgpack_test(fixture_name: &str, commands: Option<&str>) -> nu_test_support::
         .join("generate.nu");
 
     let mut outcome = None;
-    Playground::setup(&format!("msgpack test {}", fixture_name), |dirs, _| {
+    Playground::setup(&format!("msgpack test {fixture_name}"), |dirs, _| {
         assert!(
             nu!(
                 cwd: dirs.test(),