cd: update drone config file to trigger pipeline on pull request

.drone.yml

@@ -19,12 +19,12 @@ type: docker
 name: default
 
 steps:
-- name: quality-check
+  - name: quality-check
     image: git.wyatt-studio.fr/wyatt-foss/flutter-melos:2.9.0-1
     commands:
       - melos run quality-check
       - melos run publish:validate
-- name: publish
+  - name: publish
     image: git.wyatt-studio.fr/wyatt-foss/flutter-melos:2.9.0-1
     commands:
       - melos run publish:validate
@@ -34,3 +34,4 @@ trigger:
     - master
   event:
     - push
+    - pull_request