left-fixed columns covers scrollable columns abnormally when table rows are selectable；使用行选择时左侧固定列非正常遮挡滚动列。

[lchrennew]

• I have searched the issues of this repository and believe that this is not a duplicate.

Version

2.0.0-rc.4

Environment

Mac, Chrome 86, Vue3.0.2

Reproduction link

https://user-images.githubusercontent.com/1522365/102693888-ac49b900-4258-11eb-9155-012c00bed263.png

Steps to reproduce

同时启用a-table的左侧固定列和行选择
在overflow时，固定列遮挡后续列（遮挡宽度同行选择列的宽度）
具体见下图

What is expected?

固定列不会因为存在行选择列而遮挡正常列

What is actually happening?

存在遮挡

[github-actions]

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