fix: add missing proxy/functions source call

plugins/nginx-vhosts/subcommands/build-config

@@ -1,6 +1,7 @@
 #!/usr/bin/env bash
 set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
 source "$PLUGIN_CORE_AVAILABLE_PATH/common/functions"
+source "$PLUGIN_AVAILABLE_PATH/proxy/functions"
 
 nginx_build_config_cmd() {
   declare desc="build nginx config to proxy app containers from command line"