RFR: 8213619: Windows powershell build script missing environment variables

[Dean Wookey]

Hi,

Please review the fix for JDK-8213619: Windows powershell build script
missing environment variables:
https://bugs.openjdk.java.net/browse/JDK-8213619
https://github.com/javafxports/openjdk-jfx/pull/276

Dean