Vagrantfile: cygwin: Fix installation of pip.
Vagrantfile: cygwin: Fix installation of pip.

cygwin is at python-3.6 now. Use version-independent `ensurepip`
to be future-proof.

edgewood/borg,RonnyPfannschmidt/borg,RonnyPfannschmidt/borg,RonnyPfannschmidt/borg,edgewood/borg,RonnyPfannschmidt/borg,RonnyPfannschmidt/borg,edgewood/borg,edgewood/borg