Use upstream drone s3 cache plugin

.drone.yml

@@ -3,7 +3,7 @@ name: default
 
 steps:
   - name: cache-restore
-    image: r.sbruder.de/drone-s3-cache
+    image: plugins/s3-cache
     settings:
       pull: true
       endpoint: https://s3.sbruder.de
@@ -48,7 +48,7 @@ steps:
           - master
 
   - name: cache-rebuild
-    image: r.sbruder.de/drone-s3-cache
+    image: plugins/s3-cache
     settings:
       pull: true
       endpoint: https://s3.sbruder.de
@@ -61,7 +61,7 @@ steps:
         - static/episodes
 
   - name: cache-flush
-    image: r.sbruder.de/drone-s3-cache
+    image: plugins/s3-cache
     settings:
       pull: true
       endpoint: https://s3.sbruder.de