cmd/go/internal/envcmd: on unix pass script contents directly to sh -c

cmd/go/internal/envcmd: on unix pass script contents directly to sh -c

Instead of writing them to a file and executing that file.

For #59998

Change-Id: I341786926762359f67dccb475295afbbb8ed1054
Reviewed-on: https://go-review.googlesource.com/c/go/+/493555
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>