Refuse to download `desktop-galaxy-overlay` for Linux

[cybik]

Describe the bug comet allows its Linux runtime to download the Galaxy overlay.

To Reproduce Steps to reproduce the behavior:

1. Run Comet's Linux binary with the right tokens, and trigger the overlay command

Expected behavior If the comet runtime is running on Linux natively, it could be assumed that the intended platform is Linux, not Windows. Since there is no GOG Galaxy Overlay for Linux at present, it should either refuse to download it, explicitly demand that its counterpart (in this case, let's say, Heroic would be calling comet) explicitly tell it that Windows binaries are expected/wanted/requested, or clearly default to Windows with either a confirmation or a clear label that "this platform does not have a native overlay, but there's Proton so here is the other guy".

System (please complete the following information):

• OS: Linux
• Architecture: x86_64

Additional context This is probably the height of pedantry.

• If comet is run as a native Linux intermediary via Heroic or Lutris, but the game is a Windows game, then yes, downloading the overlay makes sense
• If comet is run as a native Linux intermediary for Linux-native GOG games, someone runs comet overlay, without any information to the contrary, the gamer could be made to assume the overlay will work with their native game.

It's an issue of communicating what's happening more clearly, in this case. Sorry for being excruciatingly pedantic about this one.