Set and replace "GOPATH" in the environment, because .Add-ing it there crashes if it's already there.

Under Windows, Go's stdout seems to be swallowed up unless CreateNoWindow is set to false in the ProcessStartInfo. Regrettably, when Dafny is run under lit, Go's output is still swallowed up somewhere. go figure!

Author: RustanLeino

Source/Dafny/Compiler-go.cs

@@ -3453,13 +3453,13 @@ private bool SendToNewGoProcess(string dafnyProgramName, string targetFilename,
 
       var args = string.Format("{0} \"{1}\"", verb, targetFilename);
       var psi = new ProcessStartInfo("go", args) {
-        CreateNoWindow = true,
+        CreateNoWindow = Environment.OSVersion.Platform != PlatformID.Win32NT,
         UseShellExecute = false,
         RedirectStandardInput = false,
         RedirectStandardOutput = false,
         RedirectStandardError = false,
       };
-      psi.EnvironmentVariables.Add("GOPATH", GoPath(targetFilename));
+      psi.EnvironmentVariables["GOPATH"] = GoPath(targetFilename);
 
       try {
         using (var process = Process.Start(psi)) {