package/piglit: set python executable

Without this change, piglit will detect and use python from host
resulting in build failures if python-mako, python-numpy or python-six
are not installed

Fixes:
 - http://autobuild.buildroot.org/results/8d782589b6073031626fd769982a0844f3731906



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>