Bump actions/checkout from 2 to 2.3.4

Bumps [actions/checkout](https://github.com/actions/checkout) from 2 to 2.3.4.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v2...v2.3.4)

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

.github/workflows/ci.yml

@@ -19,7 +19,7 @@ jobs:
         - os: macOS-latest
           ghc: 'latest'
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v2.3.4
     - uses: haskell/actions/setup@v1
       id: setup-haskell-cabal
       with: