Adds CI test with building the macOS electron app on Buildkite. I've also added a new pipeline in buildkite manually that uses this yaml file for steps. Here's the result of it running for this diff: [LINK](https://buildkite.com/comm/macos-build/builds/2#0186177b-721a-4eda-8db5-6fa49067c075).
Depends on D6232