Bump actions/checkout from 2 to 3

Bumps [actions/checkout](https://github.com/actions/checkout) from 2 to 3.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](https://github.com/actions/checkout/compare/v2...v3)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/build.yml

@@ -19,7 +19,7 @@ jobs:
     steps:
       - id: setup-env
         uses: cisagov/setup-env-github-action@develop
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - id: setup-python
         uses: actions/setup-python@v2
         with: