Fix CI for darwin

nix-darwin is detecting `/etc/nix/nix.conf` from the
cachix/install-nix-action and refusing to overwrite it, failing the
nix-darwin activation and therefore the rest of CI.

This commit `mv`s the existing `nix.conf` to avoid the above, and then
adds support for new-style nix commands and flakes to the nix-darwin
configuration to allow their subsequent use in CI.

It also removes the nix config from the `cachix/install-nix-action`
step, which was duplicated effort since we are blowing it away with
nix-darwin anyway.

Relevant:

- https://github.com/LnL7/nix-darwin/issues/149

flake.nix

@@ -49,7 +49,9 @@
             inherit system;
             modules = [
               ./test/integration_darwin.nix
-              darwin.darwinModules.simple
+
+              # Allow new-style nix commands in CI
+              {nix.extraOptions = "experimental-features = nix-command flakes";}
 
               home-manager.darwinModules.home-manager
               {