Update drone.yml

drone.yml

   image: debian:stable-slim
   privileged: false
   environment:
-    MAKEFLAGS: "-j 4"
+    BUILDDEPS: build-essential libc6-dev libhidapi-dev libssl-dev   
+    MAKEFLAGS: -j 4
   commands:
     - echo 'Acquire::http { Proxy "http://172.17.0.1:3142"; }' | tee -a /etc/apt/apt.conf.d/proxy
     - apt-get update
     - apt-get --yes upgrade
-    - apt-get --yes install build-essential libc6-dev libhidapi-dev libssl-dev
+    - apt-get --yes install $BUILDDEPS
     - export MAKEFLAGS
     - cd src && make 
     - cd contrib && make