remote-helpers: tests: use python directly

remote-helpers: tests: use python directly

These remote helpers use 'env python', not PYTHON_PATH, so that's where
we should check for the extensions. Otherwise, if 'python' is not
PYTHON_PATH (e.g. /usr/bin/python: Makefile's default), there will be a
mismatch between the python libraries actually accessible to the remote
helpers.

Suggested by: Torsten Bögershausen <redacted>
Signed-off-by: Felipe Contreras <redacted>
Signed-off-by: Junio C Hamano <redacted>