ci: Fix workflow failures in PRs due to slack notify step (#588)

* allow some vars to be shared

* add target branch

* remove target branch

* revert to push_request and only run slack notify when push to main

.github/workflows/ci.yml

@@ -26,7 +26,7 @@ jobs:
 
       - name: Fail if changes
         run: git diff-index --exit-code HEAD
-  
+
   test:
     runs-on: ubuntu-latest
     env:
@@ -81,7 +81,7 @@ jobs:
   notify-slack:
     runs-on: ubuntu-latest
     needs: [test]
-    if: always() && github.repository == 'linode/linodego' # Run even if integration tests fail and only on main repository
+    if: always() && github.ref == 'refs/heads/main' && github.event_name == 'push' # Run even if integration tests fail and only on main repository
 
     steps:
       - name: Notify Slack