[desktop] Allow packaging/ making the app without signing

Summary: This diff adds new yarn commands package-dev and make-dev that skip the signing step. This probably should have been done sooner but I only thought of it while adding the Buildkite CI (which is the next diff.). Now you don't have to have all the credentials to build the app for testing, and it's faster.

Test Plan: Run yarn package, yarn package-dev, yarn make, yarn make-dev on macOS and Windows and check if everything builds correctly.

Reviewers: tomek, atul

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D6232