Update CI to stop running unit tests on pushes to every branch.

Change GitHub Actions configuration so that tests are only run on (a)
pushes to master and (b) pull requests.

Author: kernelmethod

.github/workflows/CI.yml

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