Reapply "Share secrets with forks."

This reverts commit 2c0ed3a.

Author: khatchad

.github/workflows/continuous-integration.yml

@@ -2,7 +2,7 @@ name: Continuous integration
 on:
   push:
     branches: [ "master" ]
-  pull_request:
+  pull_request_target:
     branches: [ "master" ]
 concurrency:
   group: ${{ github.ref }}