nixos-install: Make compatible with Nix 2.0

The use of Nix 2.0 significantly simplifies the installer, since we
can just pass a different store URI (--store /mnt) - it's no longer
needed to set up a chroot environment for the build, and to bootstrap
Nix into the chroot.

Also, commands that need to run in the installation (namely boot
loader installation and setting a root password) are now executed
using nixos-enter.

This also removes the need for nixos-prepare-root since any required
initialisation is done by Nix or by the activation script.

nixos/modules/installer/tools/tools.nix

@@ -29,17 +29,8 @@ let
   nixos-install = makeProg {
     name = "nixos-install";
     src = ./nixos-install.sh;
-
-    inherit (pkgs) perl pathsFromGraph rsync;
     nix = config.nix.package.out;
-    cacert = "${pkgs.cacert}/etc/ssl/certs/ca-bundle.crt";
-    root_uid = config.ids.uids.root;
-    nixbld_gid = config.ids.gids.nixbld;
-    prepare_root = nixos-prepare-root;
-
-    nixClosure = pkgs.runCommand "closure"
-      { exportReferencesGraph = ["refs" config.nix.package.out]; }
-      "cp refs $out";
+    path = makeBinPath [ nixos-enter ];
   };
 
   nixos-rebuild =