Simplifications to unit testing in the CI pipeline.

- Stop running unit tests on pushes to every branch; only run them on
  (a) pushes to master or (b) pull requests.
- Stop running unit tests on nightly (since they don't provide much
  useful information and fail too often).
- Only run unit tests on Linux so that we don't go outside of GitHub
  Action's usage limits.

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 }}
@@ -11,18 +13,10 @@ jobs:
       matrix:
         version:
           - '1'    # Latest release
-          - 'nightly'
         os:
           - ubuntu-latest
-          - macOS-latest
-          - windows-latest
         arch:
           - x64
-        exclude:
-          - os: macOS-latest
-            version: 'nightly'
-          - os: windows-latest
-            version: 'nightly'
     steps:
       - uses: actions/checkout@v2
       - uses: julia-actions/setup-julia@v1