R: If R_HOME isn't set, try R RHOME command

[DavisVaughan]

I think it'd be very useful to retrieve R_HOME from the R found in PATH, if not already set. This will make the usage of Ark as a Jupyter kernel easy and frictionless.

From @lionel-

To be done as a follow up to posit-dev/ark#205

We already do this for the testing variant of start_r(), so we should be able to unify the approach with our "real" start_r()

[lionel-]

This has been the case for a while now: https://github.com/posit-dev/ark/blob/586df407ef05dd99e83770825f282fe4b47ecbab/crates/ark/src/interface.rs#L359-L374