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

The documented behavior of max-jobs defines that even with a value of 0, builds with preferLocalBuild = true still build locally. This is a fairly major policy decision that the daemon makes on behalf of the user, and is a decision the user cannot easily override.

Sometimes a machine should perform absolutely no builds, in which case their options to fully disabling builds is to either delete all the build users (not sure nix-daemon would be happy with this) or set the system to a bogus value and assume/hope nobody will use it.

Describe the solution you'd like

I propose we extend the features feature of remote building to local building, too, and use preferLocalBuild as a listed feature.

Concretely, I want to:

1. extend the nix.conf to accept a mandatory-features option
2. treat derivations where preferLocalBuild == true as if they had the "local-preferred" feature
3. add local-preferred to the default value of system-features

Backwards Compatibility

• The current behavior of max-jobs=0 can be achieved by setting mandatory-features = local-preferred and max-jobs=auto.
• Other users may want to set max-jobs=0 and assign the local-preferred feature to a specific builder which is nearby.