Some tests execute avocado directly, which uses "/usr/bin/env python" by
default. On distributions with py2 and py3 installed we might end-up
running those tests using python2 while running the main selftest by
python3.

With this change we need to make sure to pass the correct python binary
in UNITTEST_AVOCADO_CMD as it's not doing this by default (to allow any
avocado command).
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>