Do rely on existing JDK12_HOME for build tools integ tests

buildSrc/src/integTest/resources/org/elasticsearch/gradle/internal/fake_git/remote/.ci/java-versions.properties

@@ -16,6 +16,6 @@
 # specific language governing permissions and limitations
 # under the License.
 #
-ES_BUILD_JAVA=openjdk14
-ES_RUNTIME_JAVA=openjdk14
+ES_BUILD_JAVA=openjdk12
+ES_RUNTIME_JAVA=openjdk12
 GRADLE_TASK=build