Add missing env var to generic build template

Author: mark-vieira

.ci/templates.t/generic-gradle-unix.yml

@@ -10,6 +10,7 @@
           properties-content: |
             JAVA_HOME=$HOME/.java/$ES_BUILD_JAVA
             RUNTIME_JAVA_HOME=$HOME/.java/$ES_RUNTIME_JAVA
+            JAVA8_HOME=$HOME/.java/java8
             JAVA11_HOME=$HOME/.java/java11
             JAVA15_HOME=$HOME/.java/openjdk15
       - shell: |