Fix the broken OS checks

Author: cmcgee1024

Sources/Swiftly/SelfUpdate.swift

@@ -53,9 +53,7 @@ struct SelfUpdate: SwiftlyCommand {
         if let version {
 #if os(macOS)
             downloadURL = URL(string: "https://download.swift.org/swiftly/darwin/swiftly-\(version).pkg")
-#endif
-
-#if os(Linux)
+#elseif os(Linux)
 #if arch(x86_64)
             downloadURL = URL(string: "https://download.swift.org/swiftly/linux/swiftly-\(version)-x86_64.tar.gz")
 #elseif arch(arm64)