request: allow comparing examples for a specific formatter only

[acid-bong]

Is your feature request related to a problem? Please describe.

When running examples.sh, it generates examples for all formatters, which takes a lot of a) time and memory to evaluate and b) time and storage to fetch all the derivations (especially if there's a mismatch between your system's and the repo's Nixpkgs). Plus, it might fail if some programs were removed from Nixpkgs.

Describe the solution you'd like

smth like #checks.SYSTEM.examples.passthru.genemichaels

Describe alternatives you've considered

#checks.SYSTEM.formatter-genemichaels exists, but it only generates an example and doesn't check if it matches

Additional context

none?