Don't keep "disabled" substituters running

For instance, it's pointless to keep copy-from-other-stores running if
there are no other stores, or download-using-manifests if there are no
manifests.  This also speeds things up because we don't send queries
to those substituters.

src/libstore/local-store.hh

@@ -48,6 +48,8 @@ struct RunningSubstituter
     Pid pid;
     AutoCloseFD to, from, error;
     FdSource fromBuf;
+    bool disabled;
+    RunningSubstituter() : disabled(false) { };
 };