CI: pin actions by SHA

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

Author: tacaswell

.github/workflows/main.yaml

@@ -8,7 +8,7 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-python@v2
-      - uses: pre-commit/[email protected]
+      - uses: pre-commit/action@0764670bf370aab253130d534e1eda7ff497dc60 # v2.0.0
   build:
     runs-on: ubuntu-20.04
     steps:
@@ -64,7 +64,7 @@ jobs:
             ./docs/_build/html/
       - name: Publish cheatsheets and handouts
         if: ${{ github.event_name == 'push' && github.ref == 'refs/heads/master' }}
-        uses: peaceiris/actions-gh-pages@v3
+        uses: peaceiris/actions-gh-pages@373f7f263a76c20808c831209c920827a82a2847 # v3
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}
           publish_dir: ./docs/_build/html/