Introduce a build setting to detect Nightly builds. (#5852)

* Allow the AppIconBanner tool to be called with purely the build number.

And use that for the Element Pro Nightly.

* Use a build setting to determine Nightly builds.

Tools/Sources/Commands/AppIconBanner.swift

@@ -7,8 +7,8 @@ struct AppIconBanner: AsyncParsableCommand {
     @Argument(help: "Path to the input image.")
     var path: String
     
-    @Option(help: "Text for the banner.")
-    var bannerText: String
+    @Option(help: "The build number appended to the app's version.")
+    var buildNumber: Int
     
     @MainActor
     func run() async throws {
@@ -19,6 +19,9 @@ struct AppIconBanner: AsyncParsableCommand {
             throw ValidationError("Could not load the image at \(pathURL).")
         }
         
+        let version = try CI.readMarketingVersion()
+        let bannerText = "\(version) (\(buildNumber))"
+        
         let renderer = ImageRenderer(content: BannerImage(image: image,
                                                           text: bannerText))