How do I install a pip package globally instead of locally?
Why don't you try sudo with the H flag? This should do the trick.
sudo -H pip install flake8
A regular sudo pip install flake8 will try to use your own home directory. The -H instructs it to use the system's home directory. More info at https://stackoverflow.com/a/43623102/
Maybe --force-reinstall would work, otherwise --ignore-installed should do the trick.