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 <[email protected]>

Author: JP-Ellis

.github/workflows/ci.yml

@@ -1,6 +1,12 @@
 name: CI
 
-on: [push, pull_request]
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
 
 env:
   DOTNET_SKIP_FIRST_TIME_EXPERIENCE: true