Rebuild nixos and home-manager together

This makes better use of parallelization and hopefully will fail earlier
if there is an issue. Possible since

nix-community/home-manager#1099

rebuild.sh

@@ -15,8 +15,7 @@ NIXPKGS="$(nix eval --raw '(import ./nixpkgs.nix)')"
 export NIX_PATH="nixpkgs=$NIXPKGS:$NIX_PATH"
 
 # First build both systems using nix 2
-nix build '(with import <nixpkgs/nixos> { }; system)' || exit $?
-nix run nixpkgs.home-manager -c home-manager -2 build || exit $?
+nix build '(with import <nixpkgs/nixos> { }; system)' $( nix run nixpkgs.home-manager -c home-manager instantiate || exit $? ) || exit $?
 
 # Now switch both systems. If the build succeeded, this hopefully won't fail
 sudo NIXPKGS="$NIXPKGS" nixos-rebuild switch