make pfsinstall use scons if SConstruct file exist.

pfsinstall has been failing for pfs_utils for sometime because of setup.py being too modern.
It turns out that using scons does what we want and is not actually too painful.
Adding that option to our pfsinstall toolbox would be useful :
Basically just lookup for a SConstruct file and use if it exist or stick with regular eupsinstall if not.