Linux

Just adding to this for other people that don't know: you can also install most* standard Linux applications and tools on SteamOS, as long as it's available as a Flatpak, or you can install it to your user directory. Technically, you can even bypass that last caveat and install whatever you want, but you're going to have a bad time if you don't know what you're doing.

Alternatively, you can install any Linux distro on your Steam Deck if game dev isn't a good experience for you on SteamOS. In fact, Bazzite is working on a GDX (Game Developer eXperience) edition, and will likely be a solid choice once it's ready.