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

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`.

src/libstore/parsed-derivations.cc

@@ -101,6 +101,10 @@ bool ParsedDerivation::canBuildLocally(Store & localStore) const
         && !drv.isBuiltin())
         return false;
 
+    if (settings.maxBuildJobs.get() == 0
+        && !drv.isBuiltin())
+        return false;
+
     for (auto & feature : getRequiredSystemFeatures())
         if (!localStore.systemFeatures.get().count(feature)) return false;