ci: 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] <[email protected]>

.github/workflows/main.yml

@@ -19,7 +19,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: DeterminateSystems/nix-installer-action@v14
-      - uses: DeterminateSystems/magic-nix-cache-action@v7
+      - uses: DeterminateSystems/magic-nix-cache-action@v8
       - name: Run `nixci build`
         run: |
           nix run nixpkgs#nixci -- \
@@ -45,7 +45,7 @@ jobs:
         with:
           submodules: recursive
       - uses: DeterminateSystems/nix-installer-action@v14
-      - uses: DeterminateSystems/magic-nix-cache-action@v7
+      - uses: DeterminateSystems/magic-nix-cache-action@v8
       - name: Run Zola
         run: |
           nix develop --command zola build
@@ -105,7 +105,7 @@ jobs:
           fetch-depth: 0
       - uses: Swatinem/rust-cache@v2
       - uses: DeterminateSystems/nix-installer-action@v14
-      - uses: DeterminateSystems/magic-nix-cache-action@v7
+      - uses: DeterminateSystems/magic-nix-cache-action@v8
       - uses: actions-rs/cargo@v1
         with:
           command: build