fix(ci): run GH action on PRs as well

- `push` works for all PRs from my own branches, but not for PRs from forks from external contributors
  - see also agilgur5/mst-persist@c890d14

Author: agilgur5

.github/workflows/ci.yml

@@ -1,5 +1,8 @@
 name: CI
-on: [push]
+on:
+- push
+- pull_request
+
 jobs:
   ci:
     name: CI - Node ${{ matrix.node-version }}, ${{ matrix.os }}