.github: Bump cachix/cachix-action from 12 to 15

Bumps [cachix/cachix-action](https://github.com/cachix/cachix-action) from 12 to 15.
- [Release notes](https://github.com/cachix/cachix-action/releases)
- [Commits](cachix/cachix-action@v12...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/check.yml

@@ -22,7 +22,7 @@ jobs:
             access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
             experimental-features = nix-command flakes
             system = ${{ matrix.arch }}
-      - uses: cachix/cachix-action@v12
+      - uses: cachix/cachix-action@v15
         with:
           name: khanelinix
           authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'