Allow usage of Preferences.jl to configure R_HOME

[frankier]

From #480

One thing that would be a bit more robust than the current environment variable approach would be to use Preferences.jl. I assume this would be an easier change than allowing super late binding of Rhome, which would allow for it to be set from CondaPkg automatically.

I can have a go at putting together a PR if there is support for this. We could try and fall back to the environment variable approach for backwards compatibility. Would someone be willing to look at such a PR? Perhaps @palday ?

[palday]

I'm not opposed to this per se, but IIRC PythonCall and PyCall also use the ENV variable approach? That's not a strong argument, but more something to note.

One thing to think about is what happens when there is both a Preference and ENV set and they don't agree. That type of inconsistent state can lead to very confusing debugging for users (and developers 😉).

[frankier]

Fixed in 31e7859