[release-branch.go1.12] go1.12.6

Change-Id: If156d9582ad5d76589e83ac00b4fa7f3b61f1502
Reviewed-on: https://go-review.googlesource.com/c/go/+/181658
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>

VERSION

-go1.12.5
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+go1.12.6
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