CI Improvements

Merged Konstantin Pavlov requested to merge thresh/VLCKit:ci-updates-2 into master

Moves the CI to use catalina-based builders from mojave ones.

Also improves build times by providing VLC_PATH holding a directory with prebuilt tools, and tells to use as many cores as machine have (likely there is a better way though).

Similar changes are likely required in 3.0 branch if we still care about it.

Edited by Konstantin Pavlov

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