ci: bump DeterminateSystems/flake-checker-action from 5 to 8 (#24)

Bumps [DeterminateSystems/flake-checker-action](https://github.com/determinatesystems/flake-checker-action) from 5 to 8.
- [Release notes](https://github.com/determinatesystems/flake-checker-action/releases)
- [Commits](DeterminateSystems/flake-checker-action@v5...v8)

---
updated-dependencies:
- dependency-name: DeterminateSystems/flake-checker-action
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

.github/workflows/check.yml

@@ -14,7 +14,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: DeterminateSystems/nix-installer-action@v13
       - name: Check flake lockfile
-        uses: DeterminateSystems/flake-checker-action@v5
+        uses: DeterminateSystems/flake-checker-action@v8
       - name: Check flake outputs
         run: |
           nix flake check --impure --accept-flake-config --keep-going