Add option to filter build output

[Schweber]

I just started using flakes and am annoyed that the option --no-build-output isn't accepted when using flakes.

I like the build graph of nom very much but i don't want the hundreds of lines that are printed when something like xwayland is compiled.

Can you please add an option that emulates --no-build-outputs and just shows the build graph of nom (what nixos-rebuild is doing at the moment without printing it to stdout persistently)?