build(deps): bump cachix/install-nix-action from V28 to 29

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from V28 to 29. This release includes the previously tagged commit.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](cachix/install-nix-action@V28...v29)

---
updated-dependencies:
- dependency-name: cachix/install-nix-action
  dependency-type: direct:production
...

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

.github/workflows/nix.yml

@@ -12,7 +12,7 @@ jobs:
       contents: read
     steps:
       - name: Install Nix
-        uses: cachix/install-nix-action@V28
+        uses: cachix/install-nix-action@v29
         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'