fix(): execute command stored in a variable

https://stackoverflow.com/questions/4668640/how-to-execute-command-stored-in-a-variable

rootfs/usr/local/bin/run.sh

@@ -93,8 +93,8 @@ if [ -e '/flarum/app/public/assets/installed.txt' ]; then
     while read line; do
       extension="${extension}${line} "
     done < /flarum/app/extensions/list
-    composer="composer require ${extension}"
-    COMPOSER_CACHE_DIR="${CACHE_DIR}" "${composer}"
+    cmd="composer require ${extension}"
+    COMPOSER_CACHE_DIR="${CACHE_DIR}" eval $cmd
     echo "[INFO] Install extra bundled extensions: DONE."
   else
     echo "[INFO] No installed extensions"