gitlab-ci: run distcheck with multi-jobs.

3 jobs for gimp-2-10 in 56 minutes and 35 seconds (queued for 1 second)
Status Job ID Name Coverage
  Dependencies
passed #846623
deps-debian/testing

00:23:55

 
  Gimp
passed #846624
gimp-debian/testing-autotools

00:23:02

passed #846625
gimp-debian/testing-autotools-distcheck

00:32:40