ci: only lint api on api files changes

Part-of: <https://dev.funkwhale.audio/funkwhale/funkwhale/-/merge_requests/2347>

.gitlab-ci.yml

@@ -175,6 +175,9 @@ lint_api:
   allow_failure: true
   stage: lint
   dependencies: []
+  rules:
+    - if: $CI_COMMIT_BRANCH =~ /(stable|develop)/
+    - changes: [api/**/*]
 
   image: $CI_REGISTRY/funkwhale/ci/python-funkwhale-api:3.11
   before_script: