build(deps): bump DeterminateSystems/magic-nix-cache-action (#366)

Bumps [DeterminateSystems/magic-nix-cache-action](https://github.com/determinatesystems/magic-nix-cache-action) from 14 to 15.
- [Release notes](https://github.com/determinatesystems/magic-nix-cache-action/releases)
- [Commits](DeterminateSystems/magic-nix-cache-action@v14...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>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/flake-build.yaml

@@ -34,7 +34,7 @@ jobs:
       - name: Install Nix
         uses: cachix/install-nix-action@v31
 
-      - uses: DeterminateSystems/magic-nix-cache-action@v14
+      - uses: DeterminateSystems/magic-nix-cache-action@v15
 
       - name: build stable
         if: matrix.target != 'unstable'

.github/workflows/pages.yaml

@@ -38,7 +38,7 @@ jobs:
       - name: Install Nix
         uses: cachix/install-nix-action@v31
 
-      - uses: DeterminateSystems/magic-nix-cache-action@v14
+      - uses: DeterminateSystems/magic-nix-cache-action@v15
 
       - name: Build search
         # reduce max jobs to not run out of memory