Motivation / Problem
User has to jump through hoops to use a downloaded build on Intel Macs on recent versions of macOS.
OpenTTD Distribution Ltd has applied for membership of the Apple Developer Program and now has a valid code signing certificate for distribution of Mac builds. This has been added into GitHub Secrets for OpenTTD and this pull request enables the creation of signed Mac builds. The builds are then notarized by Apple with the notarization ticket stapled to the .dmg. This prevents any GateKeeper prompts from appearing when the user has downloaded a build.
Anybody who has forked the repository and runs the actions in their own branch will not be able to produce signed builds unless they provide their own certificate and developer ID. The builds can however be run successfully without signing or notarization.