Runs the build when pushing to master or a PR branch

Previously, pushing to a PR branch would cause the build
to run twice: once in response to the `push` event and once
in response to the `pull_request` event.

This would often cause one of the builds to fail, due to a
race condition when uploading the code coverage report to
Coveralls.

This commit prevents this from happening by running the build
only on pushes to the `master` branch, as well as to any branch
associated to a PR.

.github/workflows/build.yml

@@ -1,5 +1,9 @@
 name: Build
-on: [push, pull_request]
+on:
+  push:
+    branches:
+      - master
+  pull_request:
 jobs:
   build:
     name: Build and Test