all: use "unix" build tag where appropriate

all: use "unix" build tag where appropriate

For #51572

Change-Id: I23bb25b8cf1ecb9be25eb6ab9e89cd397b58b3c8
Reviewed-on: https://go-review.googlesource.com/c/go/+/572535
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>