chore(ci): fix PR from fork testing (#53)

Using `pull_request_target` instead of `pull_request` should give us
access to a token with write permission, effectivelly allowing tests
from forks to work properly.

.github/workflows/test.yml

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