Run gyp with CFG_PYTHON which is python < 3

This is required for systems where /usr/bin/python is actually python 3
auto merge of #9041 : alexcrichton/rust/use-python-2, r=thestinger

This is required for systems where /usr/bin/python is actually python 3

I already did this before, but forgot to re-port it once libuv was re-upgraded