Fix repo. fetch/checkout in PR slow CI job (#30537)

fix

Co-authored-by: ydshieh <ydshieh@users.noreply.github.com>

.github/workflows/self-new-model-pr-caller.yml

@@ -69,7 +69,7 @@ jobs:
 
       - name: Update clone
         working-directory: /transformers
-        run: git fetch && git checkout ${{ github.event.pull_request.head.sha }}
+        run: git fetch && git fetch origin pull/${{ github.event.pull_request.number }}/head:pull/${{ github.event.pull_request.number }}/merge && git checkout pull/${{ github.event.pull_request.number }}/merge
 
       - name: Reinstall transformers in edit mode (remove the one installed during docker image build)
         working-directory: /transformers