[bug report] there is a style problem when multiple table components are selected

### Element UI version
2.14.1

### OS/Browsers version
Mac Chrome Version 85.0

### Vue version
2.6.11

### Reproduction Link
https://codepen.io/hao8816/pen/vYXaQja

### Steps to reproduce
使用table组件多选时，发现最新版本checkbox的样式出现不对齐的情况

### What is Expected?
Table多选时checkbox和表头的是对齐的

### What is actually happening?
目前没有对其， 2.12.0之前的版本没有问题， 2.14.1新版本有问题