Automatically set distributed build's supportedFeatures based on that host's actual config.

Previously, we had two copies of the supported features for each host.

nix/configuration/hosts/quark/distributed_build.nix

@@ -3,11 +3,5 @@
 
   config = {
     me.distributed_build.enable = true;
-    me.distributed_build.machines.hydra = {
-      enable = true;
-      additional_config = {
-        speedFactor = 2;
-      };
-    };
   };
 }