internal/gocommand: kill gracefully
internal/gocommand: kill gracefully

We want the go command to get a chance to do its cleanups. Send it
Interrupt before Kill.

Fixes golang/go#37368.

Change-Id: Id891d2f4f85aae30d2aaa19b9c854d2346ec6e1f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/220738
Run-TryBot: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
Reviewed-by: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools