Install a vagrant package without libvirt support in nix-shell.

Given that the instructions for local development suggest using VirtualBox instead of libvirt, we can install vagrant without pulling in all the libvirt dependencies.

For me, this reduces the amount I need to download to run nix-shell from >1G to <15M. (I've carrying a patch that removed the dependency to avoid downloading that much; the measurements only include the effect of adding vagrant or the minimal vagrant to the definition.)

shell.nix

@@ -2,13 +2,16 @@ let
   release2105 = import ./nixpkgs-2105.nix { };
 in
 { pkgs ? release2105 }:
+let
+    vagrant = pkgs.vagrant.override { withLibvirt = false; };
+in
 pkgs.mkShell {
   shellHook = ''
     export NIX_PATH="nixpkgs=${pkgs.path}";
   '';
   buildInputs = [
     pkgs.morph
-    pkgs.vagrant
+    vagrant
     pkgs.jp
   ];
 }