Update backend.yml

Signed-off-by: Ettore Di Giacinto <mudler@users.noreply.github.com>

.github/workflows/backend.yml

@@ -2,7 +2,6 @@
 name: 'build backend container images'
 
 on:
-  pull_request:
   push:
     branches:
       - master
@@ -1099,4 +1098,4 @@ jobs:
         run: |
           for tag in $(echo "${{ steps.quaymeta.outputs.tags }}" | tr ',' '\n'); do
             crane push llama-cpp.tar $tag
-          done
+          done