Support for building github desktop offline

[Lunarequest]

Assuming a offline yarn mirror has been setup like here. By setting the env var OFFLINE to 1 you can build with offline. This is useful for situations where internet access is not possible, such as in a flatpak or in the Fedora build infrastructure.

[Lunarequest]

I have no idea why the windows ci broke, it seems like some sort of network issue?

[Lunarequest]

@shiftkey would you mind reviewing this pr in its altered state?

[shiftkey]

@AdvaithM I've held off on review because I wanted to suggest a different approach, and I've not had time to prove it out. Instead of this pattern:

  let result
   if (isOffline()) {
     result = spawnSync(
       'node',
       [path, '--offline', '--cwd', 'app', 'install', '--force'],
       options
     )
   } else {
     result = spawnSync(
       'node',
       [path, '--cwd', 'app', 'install', '--force'],
       options
     )
   }

I wanted to see if we could avoid duplicating the arguments and instead insert the relevant offline parameter at the appropriate place:

  const installArgs = [path, '--cwd', 'app', 'install', '--force']
  if (isOffline()) {
    installArgs.splice(1, 0, '--offline')
  }

  let result = spawnSync('node', installArgs, options)

Tomorrow I should have some more bandwidth, but hopefully that helps with some feedback in the meantime...