Use JDK 10 for 6.4 BWC builds (#32866)

The 6.4 release will officially be built with JDK 10. We should push JDK
10 down to the BWC builds for the 6.4 branch then too.

distribution/bwc/build.gradle

@@ -157,7 +157,7 @@ subprojects {
       environment('JAVA_HOME', getJavaHome(it, 8))
     } else if ("6.2".equals(bwcBranch)) {
       environment('JAVA_HOME', getJavaHome(it, 9))
-    } else if (["6.3", "6.x"].contains(bwcBranch)) {
+    } else if (["6.3", "6.4", "6.x"].contains(bwcBranch)) {
       environment('JAVA_HOME', getJavaHome(it, 10))
     } else {
       environment('JAVA_HOME', project.compilerJavaHome)