Fix nix.yaml

I talked to the cachix guy, who said our old cachix install URL got garbage collected. He gave me this one instead.

Author: isovector

.github/workflows/nix.yml

@@ -44,7 +44,7 @@ jobs:
     - if: ${{ needs.pre_job.outputs.should_skip_develop != 'true' }}
       uses: cachix/install-nix-action@v13
       with:
-        install_url: https://nixos-nix-install-tests.cachix.org/serve/lb41az54kzk6j12p81br4bczary7m145/install
+        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'
         extra_nix_config: |
           experimental-features = nix-command flakes