[Bug Report] The drop-down of el-select in Table does not appear when using :popper-append-to-body="false"

### Element UI version
2.15.7

### OS/Browsers version
Windows10 20H2， Chrome 97.0.4692.99

### Vue version
2.6.11

### Reproduction Link
https://codepen.io/weitao2515/pen/rNYjoaq

### Steps to reproduce
Table组件自定义列模板使用el-select添加属性:popper-append-to-body="false"时，下拉框没有出现

### What is Expected?
下拉框出现

### What is actually happening?
下拉框没出现