Test mirrors using Apple tool

Following https://github.com/kiwix/kiwix-apple/issues/1430, we want to make sure whatever requirements apple imposes for our macOS/iOS reader downloads, those are fulfilled by mirrors.

ATM, this means testing using `nscurl`. ⚠️ it always returns `0` so output needs to be checked instead.

This needs to run on macOS, most likely on different jobs using all the avail macOS images.

Start by locating the macOS job definitions in the repository and review how platform-specific jobs are configured. Run nscurl on the reader download mirrors, checking its output rather than its always-zero exit status. Done means the available macOS jobs test the mirrors and fail when Apple’s requirements are not met.

Written by the indexing model from the issue text.