Update CI workflow to build fine on PRs from forks

Signed-off-by: Guillaume Tardif <[email protected]>

.github/workflows/ci.yml

@@ -1,6 +1,10 @@
 name: Continuous integration
 
-on: [push]
+on:
+  push:
+    branches:
+      - main
+  pull_request:
 
 jobs:
   lint: