Reset linkmode to its default value in tool transitions (#4704)

**What type of PR is this?**

Bug fix

**What does this PR do? Why is it needed?**

`go_tool_transition` and `non_go_tool_transition` reset
`//go/config:linkmode` to `"normal"`, but the flag's default is
`"auto"`, thus resulting in unnecessary config forking.

**Which issues(s) does this PR fix?**

**Other notes for review**