Bump actions/checkout from 2.3.5 to 2.4.0 (#258)

Bumps [actions/checkout](https://github.com/actions/checkout) from 2.3.5 to 2.4.0.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v2.3.5...v2.4.0)

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -21,7 +21,7 @@ jobs:
         os: [ubuntu-latest]
     steps:
       - name: Checkout
-        uses: actions/checkout@v2.3.5
+        uses: actions/checkout@v2.4.0
 
       - name: Use Node.js
         uses: actions/setup-node@v2.4.1