ci: bump cachix/cachix-action from 13 to 15

Bumps [cachix/cachix-action](https://github.com/cachix/cachix-action) from 13 to 15.
- [Release notes](https://github.com/cachix/cachix-action/releases)
- [Commits](https://github.com/cachix/cachix-action/compare/v13...v15)

---
updated-dependencies:
- dependency-name: cachix/cachix-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/github_pages.yml

@@ -14,7 +14,7 @@ jobs:
     - uses: cachix/install-nix-action@v23
       with:
         nix_path: nixpkgs=channel:nixos-unstable
-    - uses: cachix/cachix-action@v13
+    - uses: cachix/cachix-action@v15
       with:
         name: nix-community
         authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'