unix/linux/mkall: remove race on stderr buffer

Before this CL the code would create two different exec.Command
values with Stderr pointing to a single shared bytes.Buffer.
Since we really don't need a buffer here, change it to use a pipe.

Change-Id: Ie29b872cb5bac1eb023bd4a6b650bfb80ee9c041
Reviewed-on: https://go-review.googlesource.com/c/sys/+/817360
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>