nix: add all relevant packages

With this change, users can run `nix-shell --pure --run "cargo xtask run"`, which means
they need no prerequisites on their system (except for Nix).

Author: phip1611

shell.nix

@@ -4,6 +4,8 @@ let
 in
 pkgs.mkShell rec {
   nativeBuildInputs = with pkgs; [
+    rustup
+    qemu
   ];
 
   buildInputs = with pkgs; [