Use another VM or nix install to grab the zip, extract it to a path and run the following:
genisoimage -o test2.iso -r -J steamos/
(assuming you extracted the zip to steamos/)
I've edited this post to remove most of what I said, the installer as it stands now is designed to work on UEFI compatible systems only and VirtualBox isn't compatible enough.
I'm not sure I can be bothered fixing it as the SteamOS TOS forbids distribution of modified versions.
Yes, installing an ISO created like that will work, but the way X is configured breaks all of the ttys in VirtualBox. I've got a virtual serial port ready to debug the issue but booting in recovery would work too.