CI failing due to dubious ownership of cached repo

See for example: https://gitlab.torproject.org/tpo/applications/tor-browser/-/jobs/1546220

Seems to be easily fixable. From the error message:

fatal: detected dubious ownership in repository at '/builds/tpo/applications/tor-browser'
To add an exception for this directory, call:
	git config --global --add safe.directory /builds/tpo/applications/tor-browser