Fix Environment Variable check for TeamCity Log

Was previously look for TF_BUILD which is for TFS/VSTS Environments.

Author: micsco

src/Cake.TeamCity.Module/TeamCityLog.cs

@@ -16,7 +16,7 @@ public TeamCityLog(IConsole console, Verbosity verbosity = Verbosity.Normal)
 
         public void Write(Verbosity verbosity, LogLevel level, string format, params object[] args)
         {
-            if (!string.IsNullOrWhiteSpace(System.Environment.GetEnvironmentVariable("TF_BUILD")))
+            if (!string.IsNullOrWhiteSpace(System.Environment.GetEnvironmentVariable("TEAMCITY_VERSION")))
             {
                 switch (level)
                 {
@@ -46,4 +46,4 @@ public Verbosity Verbosity
             set { _cakeLogImplementation.Verbosity = value; }
         }
     }
-}
+}