Switch to PR only

To see if the `if:` syntax with ${{ ... }} is even working correctly
for _one_ condition

.github/workflows/all_core_tests.yml

@@ -11,7 +11,7 @@ jobs:
     strategy:
       matrix:
         os: ['ubuntu-latest', 'windows-latest']
-    if: ${{ github.event_name }} != 'push' || !${{ github.event.pull_request.merged }}
+    if: ${{ github.event_name }} != 'push'
 
     steps:
     - name: Check out repo