.gitlab-ci.yml: Make the cache a hidden directory

YBD scans the current working directory for definition files, which is
problematic now that we store hundreds of cached Git repos and OSTree
repos in a subdir of the definitions repo.

To avoid wasting loads of time calling stat() on tens of thousands of
directories, the cache is now in a hidden directory.

This depends on a change to YBD that causes it to efficiently ignore
hidden directories: https://gitlab.com/baserock/ybd/commit/6676c4ac0

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