Revert "ci: Replace pull_request with pull_request_target"

This was a very bad idea since this made CI operate within the context
of the *base* branch, which meant that the tests were testing code in
the `master` branch instead of the branch that was to be merged!

.github/workflows/ci.yaml

@@ -1,6 +1,6 @@
 name: Test
 on:
-  pull_request_target:
+  pull_request:
   push:
     branches:
       - master

.github/workflows/lint.yml

@@ -1,7 +1,7 @@
 name: Lint check
 run-name: Lint code
 on:
-  pull_request_target:
+  pull_request:
   push:
     branches:
       - master