build: Unset JAVA_TOOL_OPTIONS to read java version

If env variable JAVA_TOOL_OPTIONS is set, it breaks the expected java
version output. Exp:

 Picked up JAVA_TOOL_OPTIONS: -javaagent:/usr/share/java/jayatanaag.jar
 java version "1.7.0_79"
 OpenJDK Runtime Environment (IcedTea 2.5.5) (7u79-2.5.5-0ubuntu1)
 OpenJDK 64-Bit Server VM (build 24.79-b02, mixed mode)

Unset JAVA_TOOL_OPTIONS for the purposes of reading java and javac
versions. This was encountered on an Ubuntu 15.04 system.

Change-Id: I288faaa18f8589e29cd436fc9568c1a879618c65