Make a copy of env in the recipe context
Make a copy of env in the recipe context

This will prevent changes made to shared objects by calling code
from taking effect beyond what was intended/intuitive.

Also see https://codereview.chromium.org/2751173002/
and https://chromium-review.googlesource.com/c/455783/
for context.

BUG=685746

Review-Url: https://codereview.chromium.org/2756003002

luci/recipes-py,luci/recipes-py