Add .prepare script to have r-universe automatically build new nightlies

[coatless]

Good news re r-universe (and maybe we want to carry that over to a new issue?) as I heard from @jeroen (when I asked about tricks to pre-empt a DESCRIPTION.in and fill in some way or other):

Yes you can either add a .prepare shell script or similarly bootstrap.R script that is run after checkout before CMD build https://github.com/r-universe-org/build-source/blob/master/entrypoint.sh#L44-L58

That seems like a way forward. Haven't had time to peruse his entrypoint.sh in detail yet though.

Originally posted by @eddelbuettel in #3694 (comment)

Moving our discussion to enable r-universe for the R binding out of the #3694 PR ticket.