Always install composer v2

Signed-off-by: Julius Härtl <jus@bitgrid.net>

build/autoloaderchecker.sh

@@ -9,7 +9,7 @@ then
 else
   echo "Composer not found: fetching"
   php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
-  php composer-setup.php
+  php composer-setup.php --2
   php -r "unlink('composer-setup.php');"
 fi