ci: Fix workflow events

This was found by auditing workflows based on research published here:

https://github.com/joeyparrish/workflow-cheat-sheet

The research was prompted by a workflow bug in Shaka Streamer

.github/workflows/test.yaml

@@ -17,7 +17,7 @@ jobs:
       - name: Checkout code
         uses: actions/checkout@v2
         with:
-          ref: ${{ github.event.inputs.ref || github.ref }}
+          ref: ${{ inputs.ref || github.event.head.sha }}
 
       - name: Test
         run: |