mach: fix test-tidy to not skip Cargo.lock (#32540)

PR #32465 broke the lint because it initializes FileList with a
file name (./Cargo.lock). This causes it to always return an empty
list when the `only_changed_files` parameter is `False` since `os.walk`
requires a directory and not a file.

Fixes #32530.

Signed-off-by: Mukilan Thiyagarajan <mukilan@igalia.com>

servo-tidy.toml

@@ -21,6 +21,7 @@ rand = [
 # Ignored packages with duplicated versions
 packages = [
     "bitflags",
+    "cfg_aliases",
     "cookie",
     "futures",
     "libloading",