Null out the helix access token in the test

Author: marcpopMSFT

src/Tests/msbuild.Integration.Tests/GivenDotnetInvokesMSBuild.cs

@@ -71,6 +71,7 @@ public void When_dotnet_command_invokes_msbuild_with_diag_verbosity_Then_arg_is_
 
             var cmd = new DotnetCommand(Log)
                 .WithWorkingDirectory(testInstance.Path)
+                .WithEnvironmentVariable("HelixAccessToken", "")
                 .Execute(command, "-v", "diag");
 
             cmd.Should().Pass();