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`.

Also removed cron job as all updates to `master` are checked anyway.

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

.github/workflows/build.yml

@@ -2,10 +2,11 @@ name: Code Analysis & Test
 
 on:
   push:
+    branches:
+      - master
   pull_request:
-  # Once on the first of the month at 06:00 UTC
-  schedule:
-    - cron: 0 6 1 * *
+    branches:
+      - master
 
 concurrency:
   group: ${{ github.ref }}