Form: fix style of form label in nested form (#12748)

Author: ziyoung

packages/theme-chalk/src/form.scss

@@ -123,7 +123,7 @@
   }
 
   @include when(required) {
-    .el-form-item__label:before {
+    & > .el-form-item__label:before {
       content: '*';
       color: $--color-danger;
       margin-right: 4px;