Don't let 'preferLocalBuild' override 'max-jobs=0'

[rickynils]

This resolves #3810 by changing the behavior of max-jobs = 0, so
that specifying the option also avoids local building of derivations
with the attribute preferLocalBuild = true.

A more complicated resolution was suggested by @grahamc in #3810, that also made it possible to retain the original behavior of max-jobs=0. However, it has been suggested (here and here) that just letting preferLocalbuild respect max-jobs=0 is an acceptable solution too.

[edolstra]

Can you also update this code in DerivationGoal::tryLocalBuild():

    /* Make sure that we are allowed to start a build.  If this
       derivation prefers to be done locally, do it even if
       maxBuildJobs is 0. */
    unsigned int curBuilds = worker.getNrLocalBuilds();
    if (curBuilds >= settings.maxBuildJobs && !(buildLocally && curBuilds == 0)) {
        worker.waitForBuildSlot(shared_from_this());
        outputLocks.unlock();
        return;
    }

[rickynils]

@edolstra Is there anything else than the comment that actually needs to be changed in that code? My commit changes the behavior of willBuildLocally so that it always is false if max-jobs = 0. That means buildLocally will be false in the DerivationGoal::tryLocalBuild() as long as buildMode == bmNormal. I might have missed something though?

[domenkozar]

Probably the comment needs changing too.

[edolstra]

The condition && !(buildLocally && curBuilds == 0) can be dropped.

[rickynils]

@edolstra Thanks, I've removed it now.

[edolstra]

Thanks!