patch 8.2.3174: Vim9: "legacy undo" finds "undo" variable

Problem:    Vim9: "legacy undo" finds "undo" variable.
Solution:   Do not pass lookup function to find_ex_command(). (closes #8563)

src/testdir/test_vim9_assign.vim

@@ -1632,6 +1632,13 @@ def Test_script_local_in_legacy()
     let s:legvar = 'one'
   END
   CheckScriptFailure(lines, 'E476:', 1)
+
+  edit! Xfile
+  lines =<< trim END
+      var edit: bool
+      legacy edit
+  END
+  CheckDefAndScriptSuccess(lines)
 enddef
 
 def Test_var_type_check()