using env.COMMIT_REF not github.sha for the BUILD_SHA (github#27989)

Author: peterbe

.github/workflows/azure-preview-env-deploy.yml

@@ -188,7 +188,7 @@ jobs:
           # `main-docker-cache.yml` handles updating the remote cache so we don't pollute it with PR specific code
           cache-to: ''
           build-args: |
-            BUILD_SHA=${{ github.sha }}
+            BUILD_SHA=${{ env.COMMIT_REF }}
 
       # Succeed despite any non-zero exit code (e.g. if there is no deployment to cancel)
       - name: 'Cancel any existing deployments for this PR'