Use different AppCenter secrets for macOS and iPadOS distributions

[ggoraa]

Because that info is shared between distributions, they get mixed up, and it starts working not so great to be honest

I need to update the environment setup script to also include an iPadOS-specific AppCenter secret, and then make a computed property for accessing the value, which looks like basically this:

var appCenterSecret: String {
    #if os(macOS)
    macOSAppCenterSecret
    #elseif os(iOS)
    iPadOSAppCenterSecret
    #endif
}