Update drone.yml

drone.yml

   image: debian:stable-slim
   privileged: false
   environment:
-    MAKEFLAGS="-j 4"
+    MAKEFLAGS: "-j 4"
   commands:
     - echo 'Acquire::http { Proxy "http://unino.de:3142"; }' | tee -a /etc/apt/apt.conf.d/proxy
     - apt-get update