ci: doc-build: only rebase on pull requests

Skip the rebase step if it's not a pull request. Schedule and push runs
have no base ref to rebase against anyway, currently the step is failing
and being skipped.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>

.github/workflows/doc-build.yml

@@ -50,6 +50,7 @@ jobs:
         fetch-depth: 0
 
     - name: Rebase
+      if: github.event_name == 'pull_request'
       continue-on-error: true
       env:
         BASE_REF: ${{ github.base_ref }}