ci: 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](https://github.com/actions/checkout/compare/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] <support@github.com>

.github/workflows/github_pages.yml

@@ -10,7 +10,7 @@ jobs:
         os: [ubuntu-latest]
     runs-on: ${{ matrix.os }}
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - uses: cachix/install-nix-action@v31
       with:
         nix_path: nixpkgs=channel:nixos-25.05

.github/workflows/test.yml

@@ -11,7 +11,7 @@ jobs:
         os: [ubuntu-latest, macos-latest]
     runs-on: ${{ matrix.os }}
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - uses: cachix/install-nix-action@v31
       with:
         # TODO: Adjust uninstall to not need channel

.github/workflows/update-flake.yml

@@ -11,7 +11,7 @@ jobs:
     if: github.event_name != 'schedule' || github.repository_owner == 'nix-community'
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       - name: Install Nix
         uses: cachix/install-nix-action@v31
       - name: Update flake.lock