Fix reproducible build CI job for macOS

These 3 patches should make our macOS build work again. The most important change is that we explicitly specify the target macOS version, such that osxcross doesn't have to try (and give up) with detecting our cursed macOS environment itself. There is some more info on how it tries to do this in the commit message.

I also did a cargo update cc, which I think helps on #1507 (closed), but maybe I am missing something there? It builds fine on local macOS and in the cross-environment on Linux, but this patch can be dropped if it violates anything in our release procedures.

Finally, we disallow failure from the macOS reproducible builds CI task in the future. This should fix #1394 (closed).

I have run this CI job a few times now, and each time the build have yielded the build hash 9d13b9a87f2216db0bcbc93bda1efcee4f190a1671eda96a55e4494ca59ca4fa.

Assigning this directly to @gabi-250 as she is working on the release this week, but also CC @Diziet for awareness as he has spend time on this issue too.

All in all, the macOS cross-environments remains very cursed ❤