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.

caucase/test.py

@@ -3757,6 +3757,8 @@ _caucase_sh_path = find_executable(
 )
 def _runCaucaseSh(*args):
   command = (_caucase_sh_path, ) + args
+  environ = os.environ.copy()
+  environ['CAUCASE_PYTHON'] = sys.executable
   with open(os.devnull, 'rb') as devnull:
     process = subprocess.Popen(
       command,
@@ -3764,9 +3766,7 @@ def _runCaucaseSh(*args):
       stdout=subprocess.PIPE,
       stderr=subprocess.STDOUT,
       close_fds=True,
-      env={
-        'CAUCASE_PYTHON': sys.executable,
-      },
+      env=environ,
     )
     stdout, _ = process.communicate()
     status = process.wait()