fix missing basePath (#3620)

fixes #3613 

wanted to write a test for `basePath` but have no idea at the moment to
simulate this without a reverse proxy.

Here my test setup for documentation:
```yaml
networks:
  proxy:
    driver: bridge

services:
  socket-proxy:
    privileged: true
    image: tecnativa/docker-socket-proxy:edge
    container_name: socket-proxy
    restart: unless-stopped
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock:ro
    environment:
      CONTAINERS: 1
    ports:
      - "127.0.0.1:2375:2375"
    networks:
      - proxy

  traefik:
    image: traefik:latest
    container_name: traefik
    restart: unless-stopped
    user: 1000:1000
    command:
      - "--providers.docker=true"
      - "--providers.docker.network=traefik_proxy"
      - "--providers.docker.endpoint=tcp://socket-proxy:2375"
      - "--entryPoints.http.address=:80"
      - "--global.sendAnonymousUsage=false"
      - "--log.level=INFO"
      - "--api=true"
      - "--api.dashboard=true"
#      - "--accessLog=true"
#      - "--accesslog.fields.defaultmode=keep"
#      - "--accesslog.fields.headers.defaultmode=keep"
    networks:
      - proxy
    ports:
      - "80:80"

  magicmirror:
    image: karsten13/magicmirror:develop
    container_name: mm
    restart: unless-stopped
    entrypoint:
      - sleep
      - infinity
    networks:
      - proxy
    labels:
      - "traefik.http.services.karsten13.loadbalancer.server.port=8080"
      - "traefik.http.routers.k13-http.service=karsten13"
      - "traefik.http.routers.k13-http.entrypoints=http"
      - "traefik.http.routers.k13-http.rule=Host(`localhost`) && PathPrefix(`/testbasepath`)"
      - "traefik.http.middlewares.k13-stripprefix.stripprefix.prefixes=/testbasepath"
      - "traefik.http.routers.k13-http.middlewares=k13-stripprefix"
```

js/loader.js

@@ -15,7 +15,7 @@ const Loader = (function () {
 	 * @returns {object} with key: values as assembled in js/server_functions.js
 	 */
 	const getEnvVars = async function () {
-		const res = await fetch(`${location.protocol}//${location.host}/env`);
+		const res = await fetch(`${location.protocol}//${location.host}${config.basePath}env`);
 		return JSON.parse(await res.text());
 	};