request: allow comparing examples for a specific formatter only

**Is your feature request related to a problem? Please describe.** <!-- A clear and concise description of what the problem is. Ex. I'm always frustrated when [...] --> 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** <!-- A clear and concise description of what you want to happen. --> smth like `#checks.SYSTEM.examples.passthru.genemichaels` **Describe alternatives you've considered** <!-- A clear and concise description of any alternative solutions or features you've considered. --> `#checks.SYSTEM.formatter-genemichaels` exists, but it only generates an example and doesn't check if it matches **Additional context** <!-- Add any other context or screenshots about the feature request here. --> none?