[desktop] Buildkite CI for macOS electron app

Summary:
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.

Depends on D6232

Test Plan: Try running this pipeline on this diff and check that it succeeds.

Reviewers: atul, jon

Reviewed By: atul

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6233