ci: run release commit message check only on one Gitlab

ci/prebuild.yml

@@ -39,9 +39,13 @@ release commit messages prebuild:
   stage: prebuild
   before_script: []  # nothing needed
   variables:
-    # We need to clone the repo properly so we can work with origin/master
+    # We need to clone the repo properly so we can work with origin/master.
     GIT_STRATEGY: clone
   only:
-    - /^release\//
+    refs:
+      - /^release\//
+    variables:
+      # We want this to run on gitlab.com/satoshilabs/trezor/trezor-firmware only.
+      - $CI_PROJECT_PATH_SLUG == 'satoshilabs-trezor-trezor-firmware'
   script:
     - nix-shell --run "ci/check_release_commit_messages.sh"