[flow] Add (killall flow || pkill flow || true) to cleaninstall

Summary:
When new JS dependencies are added, the flow server(s) need to be restarted to avoid inaccurate errors. In the past it's been on the developer to know this and run killall flow && flow (or the equivalent) to get their dev environment in working condition again. In this diff we just add that to the beginning of cleaninstall to take care of that for us.

killall is for macOS/Darwin (though they do have bsd pkill as well)
pkill is for Ubuntu/Debian/etc
true is for the case where there's no flow server running (eg very first run of yarn cleaninstall)

Test Plan: Ran yarn cleaninstall locally.. the pre-review CI will implicitly test things on Ubuntu/Debian

Reviewers: tomek, varun, marcin, abosh, ginsu, rohan

Reviewed By: varun

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D5223