all: use "unix" build tag where appropriate

all: use "unix" build tag where appropriate

Convert a few occurrences that were submitted after CL 389935.

For #20322
For #51572

Change-Id: I0047361916c402f8e37f515e6b09d451bd499e6e
Reviewed-on: https://go-review.googlesource.com/c/go/+/437235
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>