Add a cache purger for builtin files
Summary: Fixes T12859.
Test Plan:
- Loaded Diffusion builtin icons before recent updates, saw cached builtins.
- Ran bin/cache purge --caches builtin-file.
- Reloaded, saw new files.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T12859
Differential Revision: https://secure.phabricator.com/D18147