build(deps): bump cachix/cachix-action from 14 to 15 (#439)

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]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/integration_gauntlet.yml

@@ -21,7 +21,7 @@ jobs:
           nix_path: nixpkgs=channel:nixos-unstable
           extra_nix_config: "sandbox = false"
       - name: Cache Nix
-        uses: cachix/cachix-action@v14
+        uses: cachix/cachix-action@v15
         with:
           name: chainlink-cosmos
           authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'