circleci: ignore gitlab branches for all jobs in the validate workflow
Our previous attempt ended up breaking the CircleCI config, but
this one has been tested, see:
which shows the builds getting triggered correctly. I also pushed the same
branch under 'gitlab/alp/test', and that didn't trigger any build, as desired.
Subscribers: rwbarton, carter
Differential Revision: https://phabricator.haskell.org/D5429