Address PR feedback

- Use the right path separator char for unix.
- Contruct some of the args to crossgen outside the loop.
- Add the null check inside Command.Environment.

scripts/dotnet-cli-build/Utils/PEUtils.cs

@@ -5,7 +5,7 @@ using System.Reflection.PortableExecutable;
 
 namespace Microsoft.DotNet.Cli.Build
 {
-    public class PEUtils
+    public static class PEUtils
     {
         public static bool HasMetadata(string pathToFile)
         {