Currently, if you run:
flatpak remote-ls flathub --app-runtime=com.endlessm.Platform//eos3.2
you see no apps (correct) but hundreds of runtimes. This is inconsistent
with the documentation for the '--app-runtime' option, which says:
> List applications that use the given runtime
To fix this, default to not showing runtimes if '--app-runtime' is
given. This is consistent with the behaviour if just '--app' is
specified. If you run 'flatpak list --app-runtime=com.example.Foo
--runtime' then you get apps using that runtime, plus all other
runtimes, which seems fair enough to me.