Adds STAGING flag to conditionally use staging endpoint

AltStore/Model/Source.swift

@@ -11,7 +11,12 @@ import CoreData
 extension Source
 {
     static let altStoreIdentifier = "com.rileytestut.AltStore"
+    
+    #if STAGING
+    static let altStoreSourceURL = URL(string: "https://f000.backblazeb2.com/file/altstore-staging/apps-staging.json")!
+    #else
     static let altStoreSourceURL = URL(string: "https://cdn.altstore.io/file/altstore/apps.json")!
+    #endif
 }
 
 @objc(Source)

AltStore/Operations/FetchSourceOperation.swift

@@ -67,6 +67,11 @@ class FetchSourceOperation: ResultOperation<Source>
                     decoder.managedObjectContext = context
                     
                     let source = try decoder.decode(Source.self, from: data)
+                    
+                    #if STAGING
+                    source.sourceURL = self.sourceURL
+                    #endif
+                    
                     self.finish(.success(source))
                 }
                 catch