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)
- [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

@@ -123,7 +123,7 @@ jobs:
           chmod a+x $HOME/.cabal/bin/cabal-plan
           cabal-plan --version
       - name: checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           path: source
       - name: initial cabal.project for sdist

.github/workflows/nix-integration-test.yml

@@ -12,6 +12,6 @@ jobs:
     name: Build
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@ff7abcd0c3c05ccf6adc123a8cd1fd4fb30fb493 # v4.1.1
+      - uses: actions/checkout@93cb6efe18208431cddfb8368fd83d5badbf9bfd # v4.1.1
       - uses: DeterminateSystems/nix-installer-action@786fff0690178f1234e4e1fe9b536e94f5433196 # v20
       - run: nix flake check

.github/workflows/update-flake-lock.yml

@@ -11,7 +11,7 @@ jobs:
       contents: write
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@ff7abcd0c3c05ccf6adc123a8cd1fd4fb30fb493 # v4.1.1
+      - uses: actions/checkout@93cb6efe18208431cddfb8368fd83d5badbf9bfd # v4.1.1
         with:
           ssh-key: ${{ secrets.UPDATE_FLAKE_LOCK_SSH_KEY }}
       - uses: DeterminateSystems/nix-installer-action@786fff0690178f1234e4e1fe9b536e94f5433196 # v20