actions: update triggers. (#314)

Fix actions syntax so that checks appear in the pull request ui.

Author: jmcarp

.github/workflows/build-test.yml

@@ -1,9 +1,9 @@
 name: build & test
 on:
   push:
-    branches: main
+    branches: [main]
   pull_request:
-    branches: main
+    branches: [main]
 jobs:
   build-test:
     runs-on: ubuntu-latest