⬆️ 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](cachix/cachix-action@v13...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/main.yml

@@ -25,7 +25,7 @@ jobs:
             experimental-features = nix-command flakes
             access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
       - name: Setup cachix
-        uses: cachix/cachix-action@v13
+        uses: cachix/cachix-action@v15
         with:
           name: freed-wu
           authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"