nixos: boot/stage-1: check syntax of the generated script

nixos/modules/system/boot/stage-1.nix

@@ -248,6 +248,14 @@ let
 
     isExecutable = true;
 
+    postInstall = ''
+      echo checking syntax
+      # check both with bash
+      ${pkgs.bash}/bin/sh -n $target
+      # and with ash shell, just in case
+      ${extraUtils}/bin/ash -n $target
+    '';
+
     inherit udevRules extraUtils modulesClosure;
 
     inherit (config.boot) resumeDevice;