Now as I said that, it looks like I was slightly wrong. The pkgs.bash shebang does exist... if you're already in NixOS, making wsl -d NixOS --system --user root -- /mnt/wslg/distro/bin/nixos-wsl-recovery fail.

$  tail -n1 /bin/nixos-wsl-recovery
exec /mnt/wslg/distro//nix/store/00y13y9l90yxjaqyddkjfms0r80r1i4j-nixos-enter/bin/nixos-enter --root /mnt/wslg/distro "$@"

$  head -n1 /nix/store/00y13y9l90yxjaqyddkjfms0r80r1i4j-nixos-enter/bin/nixos-enter
#! /nix/store/xy4jjgw87sbgwylm5kn047d9gkbhsr9x-bash-5.2p37/bin/bash