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] <[email protected]>

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -19,7 +19,7 @@ jobs:
         os: [ubuntu-latest, macOS-latest]
         ghc: ['9.0', '8.10', '8.8', '8.6', '8.4', '8.2']
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v2.3.4
     - uses: haskell/actions/setup@v1
       id: setup-haskell-cabal
       with:
@@ -47,7 +47,7 @@ jobs:
     # See https://github.com/vmactions/freebsd-vm#under-the-hood.
     runs-on: macos-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v2.3.4
     - name: Build
       id: build-freebsd
       uses: vmactions/[email protected]