build(deps): bump DeterminateSystems/magic-nix-cache-action from 4 to 6

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](DeterminateSystems/magic-nix-cache-action@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>

.github/workflows/nix.yaml

@@ -23,6 +23,6 @@ jobs:
       - name: Install Nix
         uses: DeterminateSystems/nix-installer-action@v10
       - name: Setup Nix Cache
-        uses: DeterminateSystems/magic-nix-cache-action@v4
+        uses: DeterminateSystems/magic-nix-cache-action@v6
       - run: nix build
       - run: nix flake check