pkgs/top-level/release.nix: enable building the R package set

The R people don't bother providing stable URLs for their package
releases. Released versions are edited or flat-out disappear at will,
which causes us a bit of trouble, like in [1]. Hopefully, enabling R
builds on Hydra will mitigate those problems by caching the release
tarballs.

[1] https://github.com/NixOS/nixpkgs/issues/11230