ci: only set up mobile docs deploy key for main repo (#24476)

Skip this step for forks, since secrets aren't available to PRs from
forks.

Signed-off-by: JP Simard <jp@jpsim.com>

.github/workflows/docs.yml

@@ -19,6 +19,7 @@ jobs:
     - name: Generate docs
       run: mobile/docs/build.sh
     - name: Set up deploy key
+      if: github.event.pull_request.head.repo.full_name == github.repository
       uses: shimataro/ssh-key-action@193316a178ec055fcc7b018f7f76bbf64085c628
       with:
         key: ${{ secrets.ENVOY_MOBILE_WEBSITE_DEPLOY_KEY }}