I have no idea if this is a Steam or EHG problem but I’ll put this here anyway.
Installing the Linux version of the Game through Steam fails to add the 64bit launcher:
I ran through the installation twice with the same result. I also tried the options listed on the Steam support link.
I finally managed to get it installed correctly by forcing Windows compatibility updating and then removing the compatibility and updating again. Doing this downloaded the missing files. The Linux installation under Steam has worked before, so something changed recently.