Make sure to create incrementals from previously signed MAR files
When creating incremental MAR files we check right now only that MAR files from previous versions specified in
torbrowser_incremental_from are available. However, this can happen if one only has built those previous versions with MAR files still locally available. In that case, though, we'll create the wrong MAR files for macOS as those previously built MAR files do not contain the content signing bits.
We could be smarter and check whether the previously built MAR files are signed and only use them in that case for generating the incremental MAR files. Otherwise we would discard them and download the signed ones, as we do if no previously built MAR files are available locally.