Only run flake8 on Swift code

In particular, this skips the "build" directory, which is prone to
containing third-party stuff that doesn't comply with all the various
hacking/flake8/pep8 checks.

Change-Id: Ibe2240d8c6ca5947b2cac259cd7d0a1154f8a9cd
Merge "Only run flake8 on Swift code"