Add utf8 with bom (#2641)

* Change default file encoding

src/Runner.Worker/ActionNotFoundException.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Runtime.Serialization;
 
 namespace GitHub.Runner.Worker
@@ -30,4 +30,4 @@ namespace GitHub.Runner.Worker
             return $"An action could not be found at the URI '{actionUri}'";
         }
     }
-}
+}