clean_unused_caches: Allow the main function to accept `Namespace` args.

This commit will allow us to pass the arguments in the
'clean...' functions when calling the `main` function (in
`provision`). It also changes args parsing
function location to `if __name__ == "__main__"` block as
we wouldn't need it to parse args when we call the
function.

scripts/lib/clean_unused_caches.py

@@ -1,4 +1,5 @@
 #!/usr/bin/env python3
+import argparse
 import os
 import sys
 
@@ -8,8 +9,7 @@ from scripts.lib import clean_emoji_cache, clean_node_cache, clean_venv_cache, c
 from scripts.lib.zulip_tools import parse_cache_script_args
 
 
-def main() -> None:
-    args = parse_cache_script_args("This script cleans unused Zulip caches.")
+def main(args: argparse.Namespace) -> None:
     os.chdir(ZULIP_PATH)
     clean_venv_cache.main(args)
     clean_node_cache.main(args)
@@ -18,4 +18,5 @@ def main() -> None:
 
 
 if __name__ == "__main__":
-    main()
+    args = parse_cache_script_args("This script cleans unused Zulip caches.")
+    main(args)