IIRC, /bin/sh and /usr/bin/env were added due to POSIX compliance.
Pretty dogmatic, but it’s a form of global system state. Could have issues with bash_4 vs bash_5, etc. Ultimately, most programs shouldn’t assume filepaths (Yes, I’m aware shebangs needs an absolute path).
Not sure how persistent this is with nixos-rebuild.
If you want a quick hack, you could always do nix-shell -p steam; steam-run <prog>