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.

scripts/copy-from-other-stores.pl.in

@@ -16,6 +16,9 @@ foreach my $dir (@remoteStoresAll) {
     push @remoteStores, glob($dir);
 }
 
+exit if scalar @remoteStores == 0;
+print "\n";
+
 
 $ENV{"NIX_REMOTE"} = "";