ci: bump DeterminateSystems/magic-nix-cache-action from 3 to 4 (#18)

Bumps [DeterminateSystems/magic-nix-cache-action](https://github.com/determinatesystems/magic-nix-cache-action) from 3 to 4.
- [Release notes](https://github.com/determinatesystems/magic-nix-cache-action/releases)
- [Commits](https://github.com/determinatesystems/magic-nix-cache-action/compare/v3...v4)

---
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/build.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
-      - uses: DeterminateSystems/magic-nix-cache-action@v3
+      - uses: DeterminateSystems/magic-nix-cache-action@v4
       - uses: DeterminateSystems/nix-installer-action@v10
         with:
           extra-conf: |

.github/workflows/iso.yml

@@ -14,7 +14,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
-      - uses: DeterminateSystems/magic-nix-cache-action@v3
+      - uses: DeterminateSystems/magic-nix-cache-action@v4
       - uses: DeterminateSystems/nix-installer-action@v10
       - name: Build ISO
         run: |

.github/workflows/site.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
-      - uses: DeterminateSystems/magic-nix-cache-action@v3
+      - uses: DeterminateSystems/magic-nix-cache-action@v4
       - uses: DeterminateSystems/nix-installer-action@v10
       - env:
           NIXPKGS_ALLOW_UNFREE: "1"