Update directory_writer.yml

Author: izefoea

.github/workflows/directory_writer.yml

@@ -1,7 +1,8 @@
 # The objective of this GitHub Action is to update the DIRECTORY.md file (if needed)
 # when doing a git push
 name: directory_writer
-on: [push]
+on:
+  pull_request
 jobs:
   build:
     runs-on: ubuntu-latest