Try to fix string syntax

Author: vkucera

.github/workflows/o2-linter.yml

@@ -5,7 +5,7 @@ name: O2 linter
 "on": [pull_request_target, push]
 permissions: {}
 env:
-  HEAD_BRANCH: ${{ github.event_name == 'push' && github.ref || 'refs/pull/'github.event.number'/merge' }}
+  HEAD_BRANCH: ${{ github.event_name == 'push' && github.ref || 'refs/pull/'${{ github.event.number }}'/merge' }}
   BASE_BRANCH: ${{ github.event_name == 'push' && 'master' || github.event.pull_request.base.ref }}
 
 concurrency: