Use openjdk 12 in packer cache script (#40498)

Author: alpar-t

.ci/packer_cache.sh

@@ -20,5 +20,5 @@ export JAVA_HOME="${HOME}"/.java/${ES_BUILD_JAVA}
 # We are caching BWC versions too, need these so we can build those
 export JAVA8_HOME="${HOME}"/.java/java8
 export JAVA11_HOME="${HOME}"/.java/java11
-export JAVA12_HOME="${HOME}"/.java/java12
+export JAVA12_HOME="${HOME}"/.java/openjdk12
 ./gradlew --parallel clean --scan -Porg.elasticsearch.acceptScanTOS=true -s resolveAllDependencies