Bug 32805: Make creation of a downloads.json file optional

When creating update responses for nightly builds, we don't want to
create a downloads.json file, so we make that optional.

projects/release/update_responses_config.yml

 ---
 tmp_dir: '[% c("tmp_dir") %]'
+create_downloads_json: 1
 appname_marfile: tor-browser
 appname_bundle_osx: TorBrowser
 appname_bundle_linux: tor-browser

tools/update-responses/update_responses

@@ -417,6 +417,7 @@ sub write_htaccess {
 
 sub write_downloads_json {
     my ($config, @channels) = @_;
+    return unless $config->{create_downloads_json};
     @channels = keys %{$config->{channels}} unless @channels;
     foreach my $channel (@channels) {
         my $version = $config->{channels}{$channel};