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

+include:
+  - template: 'Workflows/Branch-Pipelines.gitlab-ci.yml'
+
 stages:
   - build
   - test