[bug report] the serial number of the check item is fixed, the header word displays multiple lines, and if it is stretched to one line, there will be dislocation

### Element UI version
2.13.0

### OS/Browsers version
win10/chrome Version 81.0.4044.17 (Official Build) beta (64-bit)

### Vue version
3.0.0-alpha.4

### Reproduction Link
https://codesandbox.io/s/dazzling-nobel-4x854

### Steps to reproduce
添加复选框和序号为做固定，并且有部分单元格表头超出，多行，在拉伸表头显示为一行时，出现固定列高度无法一起变化，造成行错位。

### What is Expected?
不出现错位

### What is actually happening?
固定列出现错位