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] <support@github.com>

Author: dependabot[bot]

.github/workflows/nix-flake.yml

@@ -23,7 +23,7 @@ jobs:
         extra_nix_config: |
           trusted-public-keys = hydra.iohk.io:f/Ea+s+dFdN+3Y/G+FDgSq+a5NEWhJGzdjvKNGv0/EQ= cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY= hackage-server.cachix.org-1:iw0iRh6+gsFIrxROFaAt5gKNgIHejKjIfyRdbpPYevY=
           substituters = https://hydra.iohk.io https://cache.nixos.org/ https://hackage-server.cachix.org/
-    - uses: cachix/cachix-action@v14
+    - uses: cachix/cachix-action@v15
       with:
         # https://nix.dev/tutorials/continuous-integration-github-actions#setting-up-github-actions
         name: hackage-server