Add .prepare script for CI services

[jeroen]

This would allow us to build the package on https://s-u.r-universe.dev

The problem is that rJava is one of the few R packages that needs pre-work before we can run R CMD build. We introduce a .prepare script which is run by the build server directly after cloning the repo. The script gets the directory into the conventional state where it contains the R source package, so then all the standard subsequent CI/CD tooling works.

The current script is a minimal solution, perhaps there is a more elegant way by factoring out stuff from your mkdist.

Thanks!