Revert "nixos: remove rsync from base install and add explicit path in nixos-install"

This reverts commit 582313bafe.

Removing rsync is actually pointless because nixos-install depends on
it. So if it's part of the system closure, we may as well provide it
to users.

Probably with the next Nix release we can drop the use of rsync and
use "nix copy" instead.

nixos/modules/config/system-path.nix

@@ -34,6 +34,7 @@ let
       config.programs.ssh.package
       pkgs.perl
       pkgs.procps
+      pkgs.rsync
       pkgs.strace
       pkgs.su
       pkgs.time

nixos/modules/installer/tools/nixos-install.sh

@@ -169,7 +169,7 @@ if ! NIX_DB_DIR=$mountPoint/nix/var/nix/db nix-store --check-validity @nix@ 2> /
     for i in $(@perl@/bin/perl @pathsFromGraph@ @nixClosure@); do
         echo "  $i"
         chattr -R -i $mountPoint/$i 2> /dev/null || true # clear immutable bit
-        @rsync@/bin/rsync -a $i $mountPoint/nix/store/
+        rsync -a $i $mountPoint/nix/store/
     done
 
     # Register the paths in the Nix closure as valid.  This is necessary

nixos/modules/installer/tools/tools.nix

@@ -21,7 +21,7 @@ let
     name = "nixos-install";
     src = ./nixos-install.sh;
 
-    inherit (pkgs) perl pathsFromGraph rsync;
+    inherit (pkgs) perl pathsFromGraph;
     nix = config.nix.package.out;
     cacert = "${pkgs.cacert}/etc/ssl/certs/ca-bundle.crt";
     root_uid = config.ids.uids.root;