Handle an empty app path in the packager (#13617)
Handle an empty app path in the packager (#13617)

Also make it be able to build a particular arch and platform.

I used this to build bundles to test on Windows.
keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client