ci(uffizzi): provide git ref to uffizzi preview-action

Fixes 6ec1b61a37

The JSON path at the previous PR was wrong
and should have been `$.pull_request.head.sha` instead.

Relates-to: PR #16721
Relates-to: PR #17299
Signed-off-by: Patrick Jungermann <Patrick.Jungermann@gmail.com>

.github/workflows/uffizzi-preview.yaml

@@ -70,7 +70,7 @@ jobs:
 
       - name: Read Git Ref From Event Object
         id: ref
-        run: echo "GIT_REF=${{ fromJSON(env.EVENT_JSON).head.sha }}" >> $GITHUB_ENV
+        run: echo "GIT_REF=${{ fromJSON(env.EVENT_JSON).pull_request.head.sha }}" >> $GITHUB_ENV
 
       - name: DEBUG - Print Job Outputs
         if: ${{ runner.debug }}