Restrict CI to PRs and pushes/tags on master only (#511)

Previously we were running CI for every PR twice, from the PR and push
triggers.

Author: JamesWrigley

.github/workflows/ci.yml

@@ -1,7 +1,11 @@
 name: CI
 on:
-  - push
-  - pull_request
+  push:
+    branches:
+      - master
+    tags: ['*']
+  pull_request:
+
 jobs:
   test:
     name: Julia ${{ matrix.version }} - ${{ matrix.os }} - ${{ matrix.arch }} - ${{ github.event_name }}