nixos/xonsh: source NixOS environment

Without doing that, xonsh is unusable as login shell

nixos/modules/programs/xonsh.nix

@@ -45,7 +45,32 @@ in
 
   config = mkIf cfg.enable {
 
-    environment.etc.xonshrc.text = cfg.config;
+    environment.etc.xonshrc.text = ''
+      # /etc/xonshrc: DO NOT EDIT -- this file has been generated automatically.
+
+
+      if not ''${...}.get('__NIXOS_SET_ENVIRONMENT_DONE'):
+          # The NixOS environment and thereby also $PATH
+          # haven't been fully set up at this point. But
+          # `source-bash` below requires `bash` to be on $PATH,
+          # so add an entry with bash's location:
+          $PATH.add('${pkgs.bash}/bin')
+
+          # Stash xonsh's ls alias, so that we don't get a collision
+          # with Bash's ls alias from environment.shellAliases:
+          _ls_alias = aliases.pop('ls', None)
+
+          # Source the NixOS environment config.
+          source-bash "${config.system.build.setEnvironment}"
+
+          # Restore xonsh's ls alias, overriding that from Bash (if any).
+          if _ls_alias is not None:
+              aliases['ls'] = _ls_alias
+          del _ls_alias
+
+
+      ${cfg.config}
+    '';
 
     environment.systemPackages = [ cfg.package ];