Revert not specifying building the "debian" branch; as we merge master into...

Revert not specifying building the "debian" branch; as we merge master into this branch it will (since the very introduction of the .gitlab-ci.yml file) will have it and will additionally be out of sync.

This reverts commit 568676f8.
1 job for master in 2 minutes and 47 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #222370
test

00:02:47