[Bug Report] The table that uses both "type='expand'" and "fixed='left'" in nested el-table cannot be expanded normally

### Element UI version
2.12.0

### OS/Browsers version
win10/Google Chrome 97.0.4692.99

### Vue version
2.6.10

### Reproduction Link
https://codepen.io/setwan/pen/qBpzaQq

### Steps to reproduce
1、add [fixed="left"] or [fixed="right"] to el-table-column（with table type="expand"）
2、open table expand

### What is Expected?
Form [fixed="left"] or normal form can be expanded normally

### What is actually happening?
Form [fixed="left"] or normal form can not expand normally