CI: temporarily disable (comment) all macOS jobs until we get a new build node

Our macOS build node is very old, which causes a bunch of issues:

• it is hard to maintain: the latest macOS releases cannot be installed on it
• and, because of that, no brew pre-compiled 'bottles' can be installed (brew bottles are usually built for the two latest macOS releases), which causes that all packages have to be compiled from the sources.

For the moment, until we get a new macOS build node, we comment out all macOS jobs from the CI configuration.

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This MR addresses issue #99

Cc: @tsulaia and @boudreau