fix(ci): run GH action on external PRs as well (#112)

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 https://github.com/agilgur5/mst-persist/blob/4f8b9f116d1645112ac2eb790d41f007916b9ff0/.github/workflows/ci.yml#L2

(cherry picked from commit ec4e365)

Author: agilgur5

.github/workflows/ci.yml

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