⬆️ 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](https://github.com/cachix/cachix-action/compare/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>

.github/workflows/ci.yml

@@ -493,7 +493,7 @@ jobs:
       # I could not find a way to install tree-grepper without nix
       # https://github.com/BrianHicks/tree-grepper/issues/293
       - uses: cachix/install-nix-action@v25
-      - uses: cachix/cachix-action@v14
+      - uses: cachix/cachix-action@v15
         with:
             name: tree-grepper
       - run: nix-env -if https://github.com/BrianHicks/tree-grepper/archive/refs/heads/main.tar.gz