Add utf8 with bom (#2641)

* Change default file encoding

src/Runner.Worker/JobContext.cs

@@ -1,4 +1,4 @@
-using GitHub.DistributedTask.Pipelines.ContextData;
+using GitHub.DistributedTask.Pipelines.ContextData;
 using GitHub.Runner.Common.Util;
 using GitHub.Runner.Common;
 
@@ -57,4 +57,4 @@ namespace GitHub.Runner.Worker
             }
         }
     }
-}
+}