Reduce cores used when compiling CI

Complilation seemed to be getting killed by the OOM manager

.gitlab-ci.yml

@@ -27,7 +27,7 @@ build-job:       # This job runs in the build stage, which runs first.
   script:
     - cmake --preset=ci .
     - cd /builds/DAPHNE4NFDI/build
-    - ninja
+    - ninja -j2
 
 unit-test-job:   # This job runs in the test stage.
   stage: test    # It only starts when the job in the build stage completes successfully.