Add Nix build files

[wch]

This PR adds files to build R-devel as a Nix flake.

Some notes:

• The Recommended package filenames and hashes need to be updated manually. Because CRAN continually updates the set of recommended packages for the dev version of R and removes them from their Recommended/ directory, it means that it often will not be possible to rebuild R-devel from a previous point in time. It could make sense to create a Github Action which updates the filenames and hashes very frequently, like every few hours.
• We'll want to also automate the updating of the r-source commit, commit date, SVN commit name, version string, and sha-256.
• Most of the flake.nix came from the nixpkgs R derivation here: https://github.com/NixOS/nixpkgs/blob/nixos-23.11/pkgs/applications/science/math/R/default.nix#L123.
• As of SVN commit r85619 I needed to disable string format warnings being turned into errors to get a successful build.

To update the recommended package names and hashes, run:

rsync-recommended.sh
gen-recommended-hash.sh Recommended/

The commit and push.