In google3, the Linux device is always available, and it has confused
people who run the Flutter doctor and see
"• Linux • Linux • linux-x64 • Linux" listed.

Rename the Linux device name to "Linux desktop" and the device ID to
be "linux". Make similar changes to the Windows and macOS
devices for consistency.  This is also  consistent with the web
devices.

The device ID change shouldn't be break -d usage since that does a
case-insensitive prefix match.