This requires a RUSTFLAG to be set before hand for rayon to build.
This brakes a lot of tools like rls and clippy by default and
require special configs for itnegration.
Additionally, rayon_futures is still 0.1 and not much work seem
to have gone into it. Ideally it should be replased with the tokio
runtime/threadpool.
I am not sure why, but cargo does not like it if flatpak-builder
and gnome Builder try to use the same target/ directory, even
though the environment should be almost identical.
The only difference is that Builder uses flatpak build instead of
flatpak-builder as far as I can see.
