Prevent Simplenote Electron CI from building commits without a PR

I just noticed that Buildkite will build commits that do not belong to a PR, which can result in wasted resources. Example:

- https://buildkite.com/automattic/simplenote-electron/builds/246
- https://buildkite.com/automattic/simplenote-electron/builds/247

We ought to update the `buildkite-ci` settings to prevent that.

Start by locating the repository's `buildkite-ci` settings and compare them with Buildkite builds 246 and 247. Identify which configuration permits commits outside pull requests to run, then verify that non-PR commits are skipped while pull request builds still run.

Written by the indexing model from the issue text.