chore: use inline formatting

Minor cleanup using clippy autofix.  This makes the code a bit more readable, and helps spot a few inefficiencies and possible bugs.

```
cargo clippy --fix --workspace -- -A clippy::all -W clippy::uninlined_format_args && cargo fmt
```

tests/by-util/test_cp.rs

@@ -6093,9 +6093,7 @@ fn test_cp_preserve_xattr_readonly_source() {
     let stdout = String::from_utf8_lossy(&getfattr_output.stdout);
     assert!(
         stdout.contains(xattr_key),
-        "Expected '{}' not found in getfattr output:\n{}",
-        xattr_key,
-        stdout
+        "Expected '{xattr_key}' not found in getfattr output:\n{stdout}"
     );
 
     at.set_readonly(source_file);

tests/by-util/test_csplit.rs

@@ -1457,7 +1457,7 @@ fn create_named_pipe_with_writer(path: &str, data: &str) -> std::process::Child
     nix::unistd::mkfifo(path, nix::sys::stat::Mode::S_IRWXU).unwrap();
     std::process::Command::new("sh")
         .arg("-c")
-        .arg(format!("printf '{}' > {path}", data))
+        .arg(format!("printf '{data}' > {path}"))
         .spawn()
         .unwrap()
 }

tests/by-util/test_head.rs

@@ -435,7 +435,7 @@ fn test_all_but_last_bytes_large_file_piped() {
         .len();
     scene
         .ucmd()
-        .args(&["-c", &format!("-{}", seq_19001_20000_file_length)])
+        .args(&["-c", &format!("-{seq_19001_20000_file_length}")])
         .pipe_in_fixture(seq_20000_file_name)
         .succeeds()
         .stdout_only_fixture(seq_19000_file_name);
@@ -695,7 +695,7 @@ fn test_validate_stdin_offset_bytes() {
         .len();
     scene
         .ucmd()
-        .args(&["-c", &format!("-{}", seq_19001_20000_file_length)])
+        .args(&["-c", &format!("-{seq_19001_20000_file_length}")])
         .set_stdin(file)
         .succeeds()
         .stdout_only_fixture(seq_19000_file_name);