ci: only run integration tests when there are code changes

This prevents running the integration tests, which cause load on the CI
and take time, when the change is unrelated to pmb functionality (e.g.
documentation updates).

Part-of: https://gitlab.postmarketos.org/postmarketOS/pmbootstrap/-/merge_requests/2608

.gitlab-ci.yml

@@ -127,6 +127,14 @@ deploy:
 
 .integration:
   stage: integration-test
+  rules:
+  - if: $CI_PIPELINE_SOURCE == 'merge_request_event'
+    changes:
+      - ".ci/integration_tests/**/*"
+      - "helpers/**/*"
+      - "pmb/**/*"
+      - .ci/integration.sh
+      - .gitlab-ci.yml
   before_script:
     - *global_before_scripts
     - apk upgrade -U