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-cmd-lang/src/core_commands/describe.rs

@@ -296,7 +296,7 @@ fn run(
             } else {
                 let value = stream.into_value();
                 let base_description = value.get_type().to_string();
-                Value::string(format!("{} (stream)", base_description), head)
+                Value::string(format!("{base_description} (stream)"), head)
             }
         }
         PipelineData::Value(value, ..) => {

crates/nu-cmd-lang/src/core_commands/error_make.rs

@@ -229,7 +229,7 @@ fn make_other_error(value: &Value, throw_span: Option<Span>) -> ShellError {
             error: "invalid error format.".into(),
             msg: "`$.label.start` should be smaller than `$.label.end`".into(),
             span: Some(label_span),
-            help: Some(format!("{} > {}", span_start, span_end)),
+            help: Some(format!("{span_start} > {span_end}")),
             inner: vec![],
         };
     }