chore(deps): bump cachix/install-nix-action from 14 to 15

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 14 to 15.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](https://github.com/cachix/install-nix-action/compare/v14...v15)

---
updated-dependencies:
- dependency-name: cachix/install-nix-action
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/main.yml

@@ -11,7 +11,7 @@ jobs:
       - uses: actions/checkout@v2.4.0
 
       - name: Install Nix
-        uses: cachix/install-nix-action@v14
+        uses: cachix/install-nix-action@v15
         with:
           install_url: https://nixos-nix-install-tests.cachix.org/serve/i6laym9jw3wg9mw6ncyrk6gjx4l34vvx/install
           install_options: '--tarball-url-prefix https://nixos-nix-install-tests.cachix.org/serve'