Answer by Catch22 for Setting environment variables and appending to system-wide %PATH% with a Windows batch file

I've found that placing the /M switch early does the trick, even though they want you to place it at the end.

So for instance, this works:

setx /M PATH "%PATH%;C:\AddYourPathHere"

But this doesn't:

setx PATH "%PATH%;C:\AddYourPathHere" /M