Add GitHub Desktop Installer (non Machine-Wide)

There's issues with how the machine-wide installer behaves. To avoid these issues entirely, it would be beneficial to allow users to install the .exe themselves.

https://github.com/desktop/desktop/issues/10082

Edit: Furthermore, the GitHub Desktop Deployment Tool (machine-wide installer) doesn't actually install GitHub Desktop until the next login, which would be confusing for users if this was the installer we used to offer GitHub Desktop via Software Center.

In fact, in my tests, GitHub Desktop isn't even being installed by the deployment tool. This behaviour is likely a result of our privileged identity management and access management solution. While that's on us to sort out, I can imagine other folks might encounter similar problems that could be resolved simply by using the per-user .exe instead, which might be preferable when each user is assigned their own device anyway.

That said, the machine-wide installer is still useful for multi-user environments, such as Azure Virtual Desktop session hosts.