Generate _env.cmd file instead of bat file for consistency.

The RefreshEnv.cmd script generates a _env.bat file. I changed the script to generate a cmd file for consistency. 

I was lead to make this change because I found that some organizations use group policy to block the execution of bat files in %TEMP%, but do not block the execution of cmd files. This might just be an oversight in the group policy configuration, but I figure it wouldn't hurt to use the newer "cmd" convention, especially if it isn't blocked.