gitlab-ci: move pygtk build into deps-debian/testing step.

This will make Python plug-in available for the main build step.
Also it should make the distcheck step shorter, hence hopefully will
prevent it from timing out so often.
3 jobs for gimp-2-10 in 60 minutes and 33 seconds (queued for 1 second)
Status Job ID Name Coverage
  Dependencies
passed #843973
deps-debian/testing

00:10:33

 
  Gimp
passed #843974
gimp-debian/testing-autotools

00:17:38

passed #843975
gimp-debian/testing-autotools-distcheck

00:49:59