nixos/system: disallow system.copySystemConfiguration in pure eval mode

This is already broken, so let's give it a better error message.

nixos/modules/system/activation/top-level.nix

@@ -338,6 +338,12 @@ in
 
 
   config = {
+    assertions = [
+      {
+        assertion = config.system.copySystemConfiguration -> !lib.inPureEvalMode;
+        message = "system.copySystemConfiguration is not supported with flakes";
+      }
+    ];
 
     system.extraSystemBuilderCmds =
       optionalString