Make signed .mar files for nightly builds available (instead of unsigned ones)

Our nightly download directories contain .mar files, too. But those are the unsigned ones. Ideally, however, the signed ones would be available instead (e.g. for applying a manual update in case of bugs like #40033 (closed)).

@boklm: not sure if that's the right place to file this bug but I guess tor-browser-build is the least wrong one. :)