STRY52685064: make sure plugin_dir has a value (#53)

scripts/mysql_install_db.sh

@@ -541,6 +541,7 @@ fi
 mysqld_bootstrap="${MYSQLD_BOOTSTRAP-$mysqld}"
 mysqld_install_cmd_line()
 {
+  test -z "$plugindir" && plugindir=$basedir/lib/plugin
   "$mysqld_bootstrap" $defaults $defaults_group_suffix "$mysqld_opt" --bootstrap $silent_startup\
   "--basedir=$basedir" "--datadir=$ldata" --log-warnings=0 --enforce-storage-engine="" \
   "--plugin-dir=${plugindir}" \