.github: Fix chart push on forks

Checkout the SHA instead of the ref on forks, since the ref will only
exist on the fork, not the upstream repository being checked out.

Signed-off-by: Chance Zibolski <chance.zibolski@gmail.com>

.github/workflows/push-chart-ci.yaml

@@ -48,7 +48,12 @@ jobs:
           echo ref="${{ inputs.checkout_ref }}" >> $GITHUB_OUTPUT
           echo sha="${{ inputs.checkout_ref }}" >> $GITHUB_OUTPUT
         elif [[ "${{ github.event_name }}" == "workflow_run" ]]; then
-          echo ref="${{ github.event.workflow_run.head_branch }}" >> $GITHUB_OUTPUT
+          if [[ "${{ github.event.workflow_run.head_repository.fork }}" == "true"  ]]; then
+            # use the SHA on forks since the head_branch won't exist in the upstream repository
+            echo ref="${{ github.event.workflow_run.head_sha }}" >> $GITHUB_OUTPUT
+          else
+            echo ref="${{ github.event.workflow_run.head_branch }}" >> $GITHUB_OUTPUT
+          fi
           echo sha="${{ github.event.workflow_run.head_sha }}" >> $GITHUB_OUTPUT
         elif [[ "${{ github.event_name }}" == "push" ]]; then
           echo ref="${{ github.ref }}" >> $GITHUB_OUTPUT