babeltrace2-ptest was referencing the host's python version in it's
makefiles. Whilst this could be fixed, its easier to correct the
python version being found to the target one which works much
more effectively and solves the issue.
(From OE-Core rev: 559f16d2fef4f8b04a9db1104b4a5b63f7876c68)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>