Build scripts should only use JDK_HOME, not JAVA_HOME

The issue was discovered after a fix for RT-35197.

$ JDK_HOME=/usr/java/8 gradle
 Defining Closed Properties
:buildSrc:clean
:buildSrc:generateGrammarSource
:buildSrc:compileJava
/net/lin/export/anthony/ws/g-28-twoTransparentWindows-RT-36716/rt/buildSrc/src/main/java/com/sun/scenario/effect/compiler/backend/sw/sse/SSEBackend.java:85: error: ')' expected
        Comparator<Variable> c = (v0, v1) -> v0.getName().compareTo(v1.getName());
                                    ^
/net/lin/export/anthony/ws/g-28-twoTransparentWindows-RT-36716/rt/buildSrc/src/main/java/com/sun/scenario/effect/compiler/backend/sw/sse/SSEBackend.java:85: error: ';' expected
        Comparator<Variable> c = (v0, v1) -> v0.getName().compareTo(v1.getName());
                                        ^
2 errors
:buildSrc:compileJava FAILED