Dedupe GitHub Actions workflows (#321)

This is a minor optimization of the following.

1. Only run the workflow 1x instead of 2x on a PR that's from a branch
   instead of a fork.
    * This is done by only triggering on push events when they're to the
      master branch. If you want to trigger the workflow on other
      branches, be sure to open a PR.
2. Do not run the workflow on tags.
    * Tags are made on existing commits on the master branch, which
      already has workflow coverage, per the previous point.

Optimizing this isn't a big deal for an open source project on GitHub,
where GitHub Actions usage is free. However, it is a big deal for
private repository development, where usage is limited (where I've been
doing some testing).

.github/workflows/ci.yml

@@ -1,5 +1,11 @@
 name: build
-on: [push, pull_request]
+on:
+  pull_request: {}
+  push:
+    branches:
+      - "master"
+    tags-ignore:
+      - "**"
 jobs:
   test:
     strategy: