chore: manual inline formatting

Minor manual cleanup - inlined many format args.  This makes the code a bit more readable, and helps spot a few inefficiencies and possible bugs.  Note that `&foo` in a `format!` parameter results in a 6% extra performance cost, and does not get inlined by the compiler (yet).

tests/by-util/test_du.rs

@@ -585,11 +585,7 @@ fn test_du_h_precision() {
             .arg("--apparent-size")
             .arg(&fpath)
             .succeeds()
-            .stdout_only(format!(
-                "{}\t{}\n",
-                expected_output,
-                &fpath.to_string_lossy()
-            ));
+            .stdout_only(format!("{expected_output}\t{}\n", fpath.to_string_lossy()));
     }
 }
 
@@ -659,7 +655,7 @@ fn birth_supported() -> bool {
     let ts = TestScenario::new(util_name!());
     let m = match std::fs::metadata(&ts.fixtures.subdir) {
         Ok(m) => m,
-        Err(e) => panic!("{}", e),
+        Err(e) => panic!("{e}"),
     };
     m.created().is_ok()
 }