SCSDKLoginClient's completion is called twice

We convert SCSDKLoginClient.login(from:) from a completion handler to async await here. We were getting a bunch of crashes because continuation.resume() was being called twice - meaning the completion handler was being called twice.

@MainActor
func login(from viewController: UIViewController) async throws -> Bool {
    var completed = false

    return try await withCheckedThrowingContinuation { continuation in
        SCSDKLoginClient.login(from: viewController) { success, error in
            guard !completed else {
                return
            }

            completed = true

            if let error {
                continuation.resume(throwing: error)
            } else {
                continuation.resume(returning: success)
            }
        }
    }
}