Optimize DeactivateVsoCommands regexp (#4862)

* Optimize DeactivateVsoCommands regexp

* Use internal cache + don't compile

Author: localconst

src/Agent.Sdk/Util/StringUtil.cs

@@ -291,8 +291,7 @@ public static string DeactivateVsoCommands(string input)
                 return string.Empty;
             }
 
-            var vsoRegex = new Regex("##vso", RegexOptions.IgnoreCase);
-            return vsoRegex.Replace(input, "**vso");
+            return Regex.Replace(input, "##vso", "**vso", RegexOptions.IgnoreCase);
         }
     }
 }