make: run flake8 on specific directories

[SnoopJ]

Description

Tin. Run the linter on only the files we are concerned with. This is slightly friendlier to developers who have throwaway scripts or a virtual environment (other than one in env/) in their repository directory.

Checklist

• [x] I have read CONTRIBUTING.md
• [x] I can and do license this contribution under the EFLv2
• [x] No issues are reported by make qa (runs make lint and make test)
• [x] I have tested the functionality of the things this change touches