Use pull_request_target trigger instead of pull_request (#124)

We need this in order to be able to get the secrets.

Author: llucax

.github/workflows/auto-dependabot.yaml

@@ -1,7 +1,15 @@
 name: Auto-merge Dependabot PR
 
 on:
-  pull_request:
+  # XXX: !!! SECURITY WARNING !!!
+  # pull_request_target has write access to the repo, and can read secrets. We
+  # need to audit any external actions executed in this workflow and make sure no
+  # checked out code is run (not even installing dependencies, as installing
+  # dependencies usually can execute pre/post-install scripts). We should also
+  # only use hashes to pick the action to execute (instead of tags or branches).
+  # For more details read:
+  # https://securitylab.github.com/research/github-actions-preventing-pwn-requests/
+  pull_request_target:
 
 permissions:
   contents: read