[build/setup.sh] Fix install permissions when the host has umask 0077

Test Plan:
- Add "umask 0077" to my .bashrc
- vagrant destroy
- vagrant up --provision
- Check the permissions of
  /usr/local/lib/python2.7/dist-packages/pip-1.5.6.egg-info/entry_points.txt
  inside the VM.  It should be world-readable.

Reviewers: siro

Reviewed by: siro