go: Make PRESUBMIT.py bootstrap go env in a separate step.
go: Make PRESUBMIT.py bootstrap go env in a separate step.

On new infra_presubmit builders go env is never present, and presubmit checks
all attempt to bootstrap it (concurrently), and fail.

R=iannucci@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1182783007

nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra