SFMIBanner: disable showing on linux
SFMIBanner: disable showing on linux

Disable showing the system file manager integration banner on linux, even
when it thinks it should be showing.

Issue: KBFS-4084

keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client