Bump actions/checkout from v1 to v2.3.4

Bumps [actions/checkout](https://github.com/actions/checkout) from v1 to v2.3.4.
- [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/v1...5a4ac9002d0be2fb38bd78e4b4dbde5606d7042f)

Signed-off-by: dependabot[bot] <support@github.com>
diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index dd5c44f..b6a1bb9 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -10,7 +10,7 @@
     runs-on: ubuntu-latest
     steps:
       - name: Checkout master
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2.3.4
 
       - name: Deploy docs
         uses: mhausenblas/mkdocs-deploy-gh-pages@v1.16