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] <support@github.com>

Author: dependabot[bot]

.github/workflows/build-packages.yml

@@ -21,7 +21,7 @@ jobs:
     name: "Build library ${{ matrix.pkg }} for ${{ matrix.nixpkgs_branch }} on ${{ matrix.system }}"
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: cachix/install-nix-action@v31
         with:
           extra_nix_config: |
@@ -56,7 +56,7 @@ jobs:
     name: "Build application ${{ matrix.pkg }} for ${{ matrix.nixpkgs_branch }} on ${{ matrix.system }}"
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: cachix/install-nix-action@v31
         with:
           extra_nix_config: |