R: don't recurse into the rPackages set to conserve memory and CPU time
R: don't recurse into the rPackages set to conserve memory and CPU time

The addition of CRAN has made "nix-env -qa \*" run ~30% longer than before [1].

[1] https://github.com/NixOS/nixpkgs/commit/0d264c17616646a7d68e0a465fdc0f335e0d2fb3

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