ci: bump DeterminateSystems/magic-nix-cache-action from 4 to 6 (#169)

Bumps [DeterminateSystems/magic-nix-cache-action](https://github.com/determinatesystems/magic-nix-cache-action) from 4 to 6.
- [Release notes](https://github.com/determinatesystems/magic-nix-cache-action/releases)
- [Commits](https://github.com/determinatesystems/magic-nix-cache-action/compare/v4...v6)

---
updated-dependencies:
- dependency-name: DeterminateSystems/magic-nix-cache-action
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

.github/workflows/ci.yml

@@ -39,7 +39,7 @@ jobs:
         uses: cachix/install-nix-action@v26
 
       - name: Setup cache
-        uses: DeterminateSystems/magic-nix-cache-action@v4
+        uses: DeterminateSystems/magic-nix-cache-action@v6
 
       - name: Run VM
         run: |