Automatic out-of-date facter.json detection

[mightyiam]

I think I would like NixOS activation to tell me whether my facter.json is out-of-date and perhaps even show me the differences, optionally.

[nix-config-storage]

In order to not have to run the script each time to check, I imagine caching\saving the facter.json as a derivation in some way integrating the nix-build system, I don't know the technicalities, but I'm referring to evaluation caching, and then it could recognize that there are changes without having to actually know what the changes are... And prompt you to update your facter.json

[brianmcgee]

I've been working on a pre-switch check which runs facter again and compares the output.

In doing so, I'm discovering some unstable parts of the report which I need to understand and refine.