update versions cache, fix savecache for unofficial check

Author: unitycoder

UnityLauncherPro/GetUnityUpdates.cs

@@ -42,11 +42,12 @@ public static async Task<List<UnityVersion>> FetchAll(bool useUnofficialList = f
                 allVersions = unofficialVersions.Concat(allVersions).ToList();
             }
 
-            if (newVersions.Count > 0)
+            if (newVersions.Count > 0 || (useUnofficialList && unofficialReleaseURLs.Count > 0))
             {
                 SaveCachedVersions(allVersions);
             }
 
+
             return allVersions;
         }