cleanup + temporary fix of CI

The .gitlab-ci.yml file is updated:

• the CMSSW installation is replaced with the CMake installation
• the temporary version of the installer is updated (a final fix will be necessary in the future)
• we stick at building with makefiles (Ninja may be reintroduced later)
• parallelisation is fixed
• the test of CI executables is currently disabled, as this requires CMSSW to produce a test sample