caucase.test: Propagate our environment to caucase.sh .

The test should not need to sanitise the environment of this test in
particular (if we do not trust the environment then there would be a lot
more to sanitise for the python part of the test as well), and the intent
was just to add the CAUCASE_PYTHON variable so caucase.sh runs the expected
python executable and not one possibly picked from PATH.
So copy environment, edit the copy and pass this to the caucase.sh
subprocess.