Make sure to create incrementals from previously signed MAR files

[gk]

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.

[boklm]

There is a patch for this in branch bug_26054:
​https://gitweb.torproject.org/user/boklm/tor-browser-build.git/commit/?h=bug_26054&id=b2de50ce48755190259c7d274c6ce65317ccb0b1

This branch is based on the commit for #26059, so it will have to be rebased if #26059 needs some revision.

I checked that if one of the osx64 mar files from the previous version is not code-signed, it fails with an error.

[gk]

Looks good! Cherry-picked to master (commit 0bfae091fa18b6bca05de2cac42bacac47381876).

[gk]

Backported to maint-7.5 as commit 6df6be50faee48c807e8ca257e9179998aa8bc61.