[desktop] Display a warning when trying to build macos universal

Summary:
More context in ENG-2214. There's an unresolved bug when you are trying to build a universal macos (arm + x64 in one app) version of the app.
It only happens if you already have built the app for one of these versions. This diff adds a warning message that tells you to remove these files if they exist. It also adds a helper script for removing the build files.

Test Plan:

• Build the x64 or arm version
• Try building the universal version

The warning should be displayed

• Remove the files and try building the universal version. It should succeed.

Reviewers: tomek, atul, ginsu, ashoat

Reviewed By: ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D5869