gitlab-ci: Split build into two halves
gitlab-ci: Split build into two halves

Currently we are having quite some trouble keeping up with our build volume due
to the number of configurations we have. I've split the pipeline into two
halves:

 * build, which builds just vanilla deb9-linux with make and hadrian
 * full-build, which builds everything else

This will trade-off some latency to more efficient use of our builders.

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc