CI: pin actions by SHA

This eliminates the possibility of a tag being changed under
us.

Author: tacaswell

.github/workflows/gh-pages.yml

@@ -28,7 +28,7 @@ jobs:
           make -C docs html O="-t release"
 
       - name: Deploy
-        uses: peaceiris/actions-gh-pages@v3
+        uses: peaceiris/actions-gh-pages@373f7f263a76c20808c831209c920827a82a2847 # v3
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}
           publish_dir: ./docs/build/html