CI: Run CI workflow on pull requests

Let's run the CI workflow on pull requests, so that feedback is more
immediate (well, to the degree that is possible). The only concern here
could be that we now potentially run twice when creating a pull request
and then merging it. But that seems like a tiny price to pay. If we were
to force pull request creation we could remove running on push, but I am
not sure that's worth it.

Signed-off-by: Daniel Müller <deso@posteo.net>

.github/workflows/ci.yml

@@ -1,5 +1,7 @@
 name: CI
-on: push
+on:
+  pull_request:
+  push:
 
 jobs:
   test-gnu: