don't run purenix for corefn.json files that are older than the output default.nix file

[cdepillabout]

Right now, when you run purenix, it looks for all the output/*/corefn.json files, and transpiles each of them to output/*/default.nix.

It may be possible to instead check if an output/*/default.nix file is newer than output/*/corefn.json, and not regenerate the output/*/default.nix in that case.

We should confirm that purs has similar functionality, and doesn't touch the corefn.json file when it is already newer than the input src/*.purs input file.

There was a little discussion about this in https://github.com/purenix-org/purenix/issues/22#issuecomment-939258577.