[MLGO] Switch test to invoke pytype through python

Currently this test is assuming that ~/.local/bin (or wherever the
Python binaries are installed to) is on the path. This is not a valid
assumption in most cases, and it also means we might not find the
version of pytype associated with the Python interpreter being used to
test LLVM, which could create some inconsistencies. This patch fixes
that by invoking pytype through the Python interpreter passed to lit.