gitlab ci: change to pull request builds

the server receives pull requests now directly and will test builds them with this change.

.gitlab-ci.yml

@@ -34,6 +34,7 @@ test-rust-stable:
     - master
     - schedules
     - web
+    - /^[0-9]+$/
   tags:
     - rust-stable
 
@@ -46,3 +47,8 @@ build-linux:
     - cargo build --release
   tags:
     - rust-stable
+  only:
+    - master
+    - tags
+    - web
+