package/ninja: invoke python3 explicitly to configure the package

ninja depends on python3 specifically, but the configure.py file
simply uses "env python". Where no python is selected for the target
you simply won't get a python symlink in $(HOST_DIR)/usr/bin, so the
configure.py script fails to run since it can't find "python".

Notice that in order to reproduce the issue, you must not have
python2 installed on your host machine.

Signed-off-by: Avi Shukron <avraham.shukron@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>