[bootstrap] make ultimate invocation more isolated
[bootstrap] make ultimate invocation more isolated

This should avoid the pyasn1 errors until we get vpython deployed for
recipe_engine.

R=dnj@chromium.org, phajdan.jr@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2817693004

luci/recipes-py,luci/recipes-py