CI: Relax the branch filters back. [skip ci]

CI: Relax the branch filters back. [skip ci]

Address feedback from Francois-Xavier and do not limit everyday CI to
the branches that are known-good for running build_matrix.sh, only mind
to skip the coverity_scan branch. Although this arrangement is not what
an upstream repository should use, it keeps CI simple in fork
repositories that have arbitrarily named branches with a working
build_matrix.sh.

This should also restore pull requests CI if the strict filters had
affected it.