Source plugins from $PLUGIN_AVAILABLE_PATH when not using common plugin

This will allow us to override any core plugin

plugins/build-env/pre-build-buildpack

@@ -1,7 +1,7 @@
 #!/usr/bin/env bash
 set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
 source "$PLUGIN_CORE_AVAILABLE_PATH/common/functions"
-source "$PLUGIN_CORE_AVAILABLE_PATH/config/functions"
+source "$PLUGIN_AVAILABLE_PATH/config/functions"
 
 APP="$1"; IMAGE=$(get_app_image_name $APP); BUILD_ENV=""
 verify_app_name "$APP"