Bump actions/checkout from 5 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 5 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v5...v6)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

.github/workflows/haskell-ci.yml

@@ -170,7 +170,7 @@ jobs:
           key: ${{ runner.os }}-${{ matrix.compiler }}-tools-54f36d55
           path: ~/.haskell-ci-tools
       - name: checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           path: source
       - name: initial cabal.project for sdist

.github/workflows/linters.yml

@@ -8,7 +8,7 @@ jobs:
 
     steps:
     - name: 'Checkout repository'
-      uses: actions/checkout@v5
+      uses: actions/checkout@v6
 
     - name: 'Set up cabal-gild'
       uses: tfausak/cabal-gild-setup-action@v2
@@ -21,7 +21,7 @@ jobs:
 
     steps:
     - name: 'Checkout repository'
-      uses: actions/checkout@v5
+      uses: actions/checkout@v6
 
     - name: 'Run fourmolu'
       uses: haskell-actions/run-fourmolu@v11
@@ -31,7 +31,7 @@ jobs:
 
     steps:
     - name: 'Checkout repository'
-      uses: actions/checkout@v5
+      uses: actions/checkout@v6
 
     - name: 'Set up hlint'
       uses: haskell-actions/hlint-setup@v2