Dirty hack workaround how to upload L4D2 workshop stuff from Linux:
- Download and unpack Crowbar to some folder
- In same folder create file steam_appid.txt which just contains 550
- Add crowbar.exe as non-Steam game
- In it's properties, enable Compatibility, set to Proton Experimental
- Launch from Steam, do stuff, close.
For other games it's all same but type corresponding steam id in steam_appid.txt