fix dpkg build and make check on trusty
Change run-pyflakes to not run python2 pyflakes on tests/vmtests.
On trusty's version of pyflakes 'make check' will fail with:
Running: pyflakes curtin/ tests/
tests/vmtests/__init__.py:221: undefined name 'FileNotFoundError'
Interestingly, xenial's pyflakes is ok with that.
The vmtests/ code is not intended to be python2, so
there is no reason to run pyflakes for python2 on that.