Bump DeterminateSystems/magic-nix-cache-action from 9 to 15

Bumps [DeterminateSystems/magic-nix-cache-action](https://github.com/determinatesystems/magic-nix-cache-action) from 9 to 15.
- [Release notes](https://github.com/determinatesystems/magic-nix-cache-action/releases)
- [Commits](DeterminateSystems/magic-nix-cache-action@v9...v15)

---
updated-dependencies:
- dependency-name: DeterminateSystems/magic-nix-cache-action
  dependency-version: '15'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

.github/workflows/cachix-upload.yml

@@ -52,7 +52,7 @@ jobs:
         if: matrix.machine.os == 'ubuntu-latest'
         uses: docker/setup-qemu-action@v4
       - name: Enable Github Cache for Nix store
-        uses: DeterminateSystems/magic-nix-cache-action@v9
+        uses: DeterminateSystems/magic-nix-cache-action@v15
       - name: Enable Cachix binary cache
         uses: cachix/cachix-action@v15
         with: