CLI: use JDK 1.6 to compile the "kotlin" launcher script

"kotlin" is used to run programs compiled by "kotlinc" which supports
emitting JVM bytecode 1.6, so it should also be runnable on JDK 1.6

build.xml

@@ -26,7 +26,6 @@
     <property name="tools.jar" value="${env.JDK_18}/lib/tools.jar"/>
 
     <property name="java.target" value="1.8"/>
-    <property name="java.target.1.6" value="1.6"/>
 
     <condition property="bootstrap.or.local.build" value="true" else="false">
         <or>
@@ -306,6 +305,8 @@
         <cleandir dir="${output}/classes/runner"/>
         <kotlinc output="${output}/classes/runner">
             <src location="${basedir}/compiler/cli/cli-runner/src"/>
+            <compilerarg line="-jvm-target 1.6"/>
+            <compilerarg line="-jdk-home ${env.JDK_16}"/>
         </kotlinc>
 
         <local name="runtime.jar"/>
@@ -863,7 +864,7 @@
             </java>
 
             <javac2 srcdir="${toString:src.dirset}" destdir="@{output}" debug="true" debuglevel="lines,vars,source"
-                    includeAntRuntime="false" source="${java.target.1.6}" target="${java.target.1.6}">
+                    includeAntRuntime="false" source="1.6" target="1.6">
                 <skip pattern="kotlin/Metadata"/>
                 <classpath>
                     <path refid="classpath.path"/>