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/nix-ci.yml

@@ -12,7 +12,7 @@ jobs:
       - uses: cachix/install-nix-action@V27
         with:
           github_access_token: ${{ secrets.GITHUB_TOKEN }}
-      - uses: cachix/cachix-action@v14
+      - uses: cachix/cachix-action@v15
         with:
           name: galois
           authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'