Use 'composer self-update --1' to prevent unintentional update to Composer 2

Author: sebastianbergmann

build.xml

@@ -25,6 +25,7 @@
 
         <exec executable="${basedir}/tools/composer" taskname="composer">
             <arg value="self-update"/>
+            <arg value="--1"/>
         </exec>
     </target>
 </project>