clean up offload-build artifacts

It turns out that offload-build keeps build artifacts in ~/.cache which it does not clean up. As some home directories have grown to ~240GB. Devtools should get a systemd service/timer to remove leftover cache files after X days, but for now we'll just add a script/service/timer for it.

find .cache/offload-build/ -mtime +15 -name '*.pkg.tar*' -delete