build(deps): bump cachix/cachix-action from 14 to 15

Bumps [cachix/cachix-action](https://github.com/cachix/cachix-action) from 14 to 15.
- [Release notes](https://github.com/cachix/cachix-action/releases)
- [Commits](cachix/cachix-action@v14...v15)

---
updated-dependencies:
- dependency-name: cachix/cachix-action
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

.github/workflows/build.yml

@@ -57,7 +57,7 @@ jobs:
     - name: Show nixpkgs version
       run: nix-instantiate --eval -E '(import <nixpkgs> {}).lib.version'
     - name: Setup cachix
-      uses: cachix/cachix-action@v14
+      uses: cachix/cachix-action@v15
       # Don't replace <YOUR_CACHIX_NAME> here!
       if: ${{ matrix.cachixName != '<YOUR_CACHIX_NAME>' }}
       with: