Annotate LockedFile with #[must_use]

Standard lock guards have the same annotation, because creating them
without binding them to a local variable is almost always a mistake.

crates/uv-fs/src/lib.rs

@@ -601,6 +601,7 @@ pub fn is_virtualenv_base(path: impl AsRef<Path>) -> bool {
 
 /// A file lock that is automatically released when dropped.
 #[derive(Debug)]
+#[must_use]
 pub struct LockedFile(fs_err::File);
 
 impl LockedFile {