Let `nix flake check` keep going when keep-going is set

When the `keep-going` option is set to `true`, make `nix flake check`
continue as much as it can before failing.

The UI isn’t perfect as-it-is as all the lines currently start with a
mostly useless `error (ignored): error:` prefix, but I’m not sure what
the best output would be, so I’ll leave it as-it-is for the time being

(This is a bit hijacking the `keep-going` flag as it’s supposed to be a
build-time only thing. But I think it’s faire to reuse it here).

Fix https://github.com/NixOS/nix/issues/4450