Change ref for head_branch (DataDog#21646)

Author: lucia-sb

.github/workflows/measure-disk-usage.yml

@@ -41,7 +41,7 @@ jobs:
         --commit ${{ github.event.workflow_run.head_sha }} \
         --format json"
 
-        if [ "${{ github.event.workflow_run.event }}" = "push" ] && [ "${{ github.ref_name }}" = "master" ]; then 
+        if [ "${{ github.event.workflow_run.event }}" = "push" ] && [ "${{ github.event.workflow_run.head_branch }}" = "master" ]; then 
           cmd="$cmd --to-dd-key ${{ secrets.DD_API_KEY }}"
         fi