Bump DeterminateSystems/magic-nix-cache-action from 7 to 8

Bumps [DeterminateSystems/magic-nix-cache-action](https://github.com/determinatesystems/magic-nix-cache-action) from 7 to 8.
- [Release notes](https://github.com/determinatesystems/magic-nix-cache-action/releases)
- [Commits](DeterminateSystems/magic-nix-cache-action@v7...v8)

---
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.yml

@@ -15,7 +15,7 @@ jobs:
  - uses: cachix/install-nix-action@v27
  with:
  nix_path: nixpkgs=channel:nixos-unstable
- - uses: DeterminateSystems/magic-nix-cache-action@v7
+ - uses: DeterminateSystems/magic-nix-cache-action@v8
  - uses: cachix/cachix-action@v15
  with:
  name: pdh