Don't execute unnecessary build jobs in CI pipeline

With this merge request, the theodolite-benchmark project as well as the SLO checkers are only built if something has changed in the corresponding directories.

Fix #218 (closed).

.gitlab-ci.yml

@@ -322,21 +322,35 @@ test-slo-checker-lag-trend:
   stage: test
   needs: []
   image: python:3.7-slim
-  script:
+  before_script:
     - cd slo-checker/record-lag
+  script:
     - pip install -r requirements.txt
     - cd app
     - python -m unittest
+  rules:
+    - if: $CI_COMMIT_TAG
+    - changes:
+      - slo-checker/record-lag/**/*
+    - when: manual
+      allow_failure: true
 
 test-slo-checker-dropped-records-kstreams:
   stage: test
   needs: []
   image: python:3.7-slim
-  script:
+  before_script:
     - cd slo-checker/dropped-records
+  script:
     - pip install -r requirements.txt
     - cd app
     - python -m unittest
+  rules:
+    - if: $CI_COMMIT_TAG
+    - changes:
+      - slo-checker/dropped-records/**/*
+    - when: manual
+      allow_failure: true
 
 deploy-slo-checker-lag-trend:
   stage: deploy