Split the macOS installer build to support macOS 10.11

Issues

• Our FinderSync extension is written in Swift 4.
• Swift 4 requires macOS 10.12.4+ and XCode 9+.
• PyInstaller needs to build the installer in macOS 10.11 if we want to support this version and later.

Rejected ideas

• Rewrite the FinderSync extenstion in Swift 3.1: will take time, energy and the action would be a bad move (regression, no more future proof).
• Move out from PyInstaller: not even a possibility, there is no alternative and we bet big on it.

Validated idea

• Build the extension in one of our mac-drive-* slave (macOS 10.12).
• Stash the resulting NuxeoFinderSync.appex file.
• Retrieve the file in the Drive-packages job running on TWANG.
• Sign the extension and proceed to the rest of the installater generation.