Refactor nixos-install to separate out filesystem build logic

The key distinction I'm drawing is that there's a component that deals
with the store of the machine being built, and another component for
the store building it. The inner part of it assumes nothing from the
builder (doesn't need chroot or root powers) so it can run comfortably
inside a Nix build, as well as nixos-rebuild. I have some upcoming work
that will use that to significantly speed up and streamline image builds
for NixOS, especially on virtualized hosts like EC2, but it's also a
reasonable speedup on native hosts.

nixos/modules/installer/tools/tools.nix

@@ -4,7 +4,6 @@
 { config, pkgs, modulesPath, ... }:
 
 let
-
   cfg = config.installer;
 
   makeProg = args: pkgs.substituteAll (args // {
@@ -17,6 +16,14 @@ let
     src = ./nixos-build-vms/nixos-build-vms.sh;
   };
 
+  nixos-prepare-root = makeProg {
+    name = "nixos-prepare-root";
+    src = ./nixos-prepare-root.sh;
+
+    nix = pkgs.nixUnstable;
+    inherit (pkgs) perl pathsFromGraph rsync utillinux coreutils;
+  };
+
   nixos-install = makeProg {
     name = "nixos-install";
     src = ./nixos-install.sh;
@@ -26,6 +33,7 @@ let
     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]; }
@@ -69,6 +77,7 @@ in
 
     environment.systemPackages =
       [ nixos-build-vms
+        nixos-prepare-root
         nixos-install
         nixos-rebuild
         nixos-generate-config
@@ -77,7 +86,7 @@ in
       ];
 
     system.build = {
-      inherit nixos-install nixos-generate-config nixos-option nixos-rebuild;
+      inherit nixos-install nixos-prepare-root nixos-generate-config nixos-option nixos-rebuild;
     };
 
   };