We build and run executables in the work directory,
and some users have $TMPDIR set noexec.

Fixes #8451.

Change-Id: I76bf2ddec84e9cb37ad9a6feb53a1a84b47aa263
Reviewed-on: https://go-review.googlesource.com/75475
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>