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/download-using-manifests.pl.in

@@ -22,6 +22,8 @@ my $curl = "$Nix::Config::curl --fail --location --insecure";
 
 # Open the manifest cache and update it if necessary.
 my $dbh = updateManifestDB();
+exit 0 unless defined $dbh; # exit if there are no manifests
+print "\n";
 
 
 # $hashCache->{$algo}->{$path} yields the $algo-hash of $path.