Fix the download-commit test to actually look for a commit (#330)

Previously, `github.event.workflow_run.head_sha` would expand to an
empty string, when the workflow isn't run due to a workflow_run
event (manually triggered or invoked by another workflow).

Use `github.sha` instead of `github.event.workflow_run.head_sha`,
for the commit hash that triggered the workflow.

Author: mstorsjo

.github/workflows/download.yml

@@ -84,7 +84,7 @@ jobs:
           workflow: upload.yml
           name: artifact
           path: artifact
-          commit: ${{ github.event.workflow_run.head_sha }}
+          commit: ${{ github.sha }}
       - name: Test
         run: cat artifact/sha | grep $GITHUB_SHA
   download-multiple: