packaging: kill only the main Keybase process in run_keybase

[upamanyus]

On Linux, executing run_keybase while Keybase is already running causes electron to crash consistently (because e.g. the electron/chromium gpu-process gets killed). This fixes that by more carefully killing only the main electron/Keybase process.

A previous PR (https://github.com/keybase/client/pull/17463) tried to fix this, but it used pgrep 'Keybase$' which checks for matches against the process name. Adding -f makes it match against the full command line arguments.