patch 8.2.4357: sticky command modifiers are too sticky

Problem:    sticky command modifiers are too sticky.
Solution:   Do not apply command modifiers to a sourced script. (closes #9751)

src/testdir/test_vim9_cmd.vim

@@ -106,6 +106,18 @@ def Test_cmdmod_execute()
   unlet b:undo
   unlet g:undone
   unlet g:undtwo
+
+  # "legacy" does not apply to a loaded script
+  lines =<< trim END
+      vim9script
+      export var exported = 'x'
+  END
+  writefile(lines, 'Xvim9import.vim')
+  lines =<< trim END
+      legacy exe "import './Xvim9import.vim'"
+  END
+  v9.CheckScriptSuccess(lines)
+  delete('Xvim9import.vim')
 enddef
 
 def Test_edit_wildcards()