build/env: change armhf builder from stretch to jessie

The far superior linux distro of champions.

Updates golang/go#17105

Change-Id: I5ea0cd2361753f61bb74bf3d4dea6c181f1427fa
Reviewed-on: https://go-review.googlesource.com/29687
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
4 files changed