Update CI.yml: don't do all tests twice on PRs

We only need to build for `push:` on master/main and tags/releases.

Copied from https://github.com/JuliaLang/Example.jl/blob/master/.github/workflows/ci.yml

Author: NHDaly

.github/workflows/CI.yml

@@ -1,7 +1,9 @@
 name: CI
 on:
-  - push
-  - pull_request
+  push:
+    branches: [main, master]
+    tags: ["*"]
+  pull_request:
 jobs:
   test:
     name: Julia ${{ matrix.version }} - ${{ matrix.os }} - ${{ matrix.arch }} - ${{ github.event_name }}