.gitlab-ci.yml: fix to build against stable when being on stable
.gitlab-ci.yml: fix to build against stable when being on stable

MrTheodor/csg,MrTheodor/csg,votca/csg,votca/csg,MrTheodor/csg,votca/csg,MrTheodor/csg,MrTheodor/csg,votca/csg