Add environment variable CHOPS_GIT_VERSION
Add environment variable CHOPS_GIT_VERSION

The variable is filled with the Chrome Appengine Version by Spinnaker.

https://critique-ng.corp.google.com/cl/340614618 - Fill the variable.
go/spinnakergit - design doc.

Bug:b/138261681
Change-Id: I089f134b2d07f05fcdfe5dc6b8d8de5cafea5074
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2517217
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Commit-Queue: Ola Karlsson <595fa75913cec2db4957d2d72dd47cbe308f512b@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py