chore(ci): prevent duplicate ci triggers

Triggering on all 'push' and 'pull_request' events will result in
duplicated CI runs in PRs (due to both conditions being met).

Instead, trigger only on pushes to `master`, or updates to PRs
targetting `master`.

Signed-off-by: JP-Ellis <josh@jpellis.me>

.github/workflows/build-and-test.yml

@@ -2,8 +2,11 @@ name: Build and test
 
 on:
   push:
+    branches:
+      - master
   pull_request:
-    branches: [master]
+    branches:
+      - master
 
 jobs:
   build-and-test-osx: