RFR: JDK-8248427: jpackage jtreg BasicTest.testTemp() test fails on Windows

[Alexey Semenyuk]

Please review fix [2] for jpackage bug [1].

Makes value of `JpIcon` wix variable absolute path. This fixes a 
regression introduced by the fix of [3] issue.

- Alexey

[1] https://bugs.openjdk.java.net/browse/JDK-8248427

[2] http://cr.openjdk.java.net/~asemenyuk/8248427/webrev.00

[3] https://bugs.openjdk.java.net/browse/JDK-8246212