[CI] Limit builds to master branch

.drone.yml

@@ -6,6 +6,12 @@ steps:
  image: silex/emacs:27-alpine
  commands:
  - emacs --script export.el
+ when:
+ branch:
+ - master
+ event:
+ exclude:
+ - pull_request
 
 - name: deploy
  image: appleboy/drone-scp