[Bug Report] 使用dart-sass打包element icon出现乱码

### Element UI version
2.15.7

### OS/Browsers version
macos big sur 11.2 / chrome版本 99.0.4844.83（正式版本） (x86_64)

### Vue version
2.6.10

### Reproduction Link
https://codepen.io/ziyoung/pen/LKNBqB

### Steps to reproduce
直接yarn build

### What is Expected?
图标正确显示

### What is actually happening?
el-icon-potato-strips:before{content:""}.el-icon-milk-tea:before{content:""}.el-icon-ice-drink:before{content:""}.el-icon-ice-tea:before{content:""}.el-icon-coffee:before{content:""}.el-icon-orange:before{content:""}.el-icon-pear:before{content:""}.el-icon-apple:before{content:""}.el-icon-cherry:before{content:""}.el-icon-watermelon:before{content:""}.el-icon-grape:before{content:""}.