generate-R-packages.R fails without these changes.
Without `cacert`, all wget/curl calls will fail
with an error about no valid certificates.
Without `nix`, calling `nix-hash` fails.
An impure nix-shell can get away with not adding
these if the system NIXPKGS version is the same
as the version of NIXPKGS used
with `nix-shell generate-shell.nix`
BioConductor releases are tied to specific R releases.
Updating the package set now automatically selects the
correct version of bioconductor to match the current version of
R.
