[TODO] Build libssh under macOS instead of using brew

[webknjaz]

This would allow us to control the version of libssh (it's currently what brew ships and that's it — they remove old versions when adding new ones, hence no control at all).

Also, having control over the building allows us to single-source libssh version for manulinux+macos instead of hoping that they are the same.

[webknjaz]

With the sunset of macos-13 in CI, this is going to be more important to solve soon: https://github.blog/changelog/2025-07-11-upcoming-changes-to-macos-hosted-runners-macos-latest-migration-and-xcode-support-policy-updates/#macos-13-is-closing-down.