patch 8.2.2809: Vim9: :def function compilation fails when using :legacy

Problem:    Vim9: :def function compilation fails when using :legacy.
Solution:   Reset CMOD_LEGACY when compiling a function. (closes #8137)
diff --git a/src/testdir/test_vim9_func.vim b/src/testdir/test_vim9_func.vim
index 230de20..5bfad6b 100644
--- a/src/testdir/test_vim9_func.vim
+++ b/src/testdir/test_vim9_func.vim
@@ -2167,10 +2167,20 @@
 
 def Test_legacy_lambda()
   legacy echo {x -> 'hello ' .. x}('foo')
+
   var lines =<< trim END
       echo {x -> 'hello ' .. x}('foo')
   END
   CheckDefAndScriptFailure(lines, 'E720:')
+
+  lines =<< trim END
+      vim9script
+      def Func()
+        echo (() => 'no error')()
+      enddef
+      legacy call s:Func()
+  END
+  CheckScriptSuccess(lines)
 enddef
 
 def DoFilterThis(a: string): list<string>