Bump actions/checkout from 2 to 4 (#54)

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

Author: dependabot[bot]

.github/workflows/main.yml

@@ -10,7 +10,7 @@ jobs:
         ghc: ['8.8', '8.10']
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - name: Setup Haskell
         uses: haskell/actions/setup@v2
         with: