Still a bit unsure if this is the right way to do it, but it at least
seems to be working good enough:

    $ ansible-galaxy collection list -p ./collections | grep sigsum
    sigsum.ansible                           1.0.4
    # /home/rgdd/src/git.sigsum.org/ansible/collections/ansible_collections
    sigsum.ansible                           1.1.0

I'm a bit unsure how to silence the global installs.