Limit push builds to the main branch (#527)

Commits to a Pull Request branch have been triggering a build twice.
That is happening because a `push` to a pull request is both a `push`
and a `pull_request` activity.

This commit fixes this issue by limiting the `push` trigger to the
`main` branch, ensuring that merged Pull Requests will run linting and
tests on the main branch, and that any Pull Request activity will also
trigger a build.

Author: neilvcarvalho

.github/workflows/build.yml

@@ -1,7 +1,8 @@
 name: Build
 on:
-  - push
-  - pull_request
+  pull_request:
+  push:
+    branches: [ main ]
 
 jobs:
   build: