Description:

`nix-channel --update` fails with the following error on a system that has never before run nix and was following the wiki:

```
$ nix-channel --update --verbose
downloading 'https://nixos.org/channels/nixpkgs-unstable'...
unpacking channels...
while setting up the build environment: executing '/usr/bin/bash': No such file or directory
builder for '/nix/store/mh8hl6c7gyyqrnzrln4j2jxg79n32slf-nixpkgs-21.11pre300283.f930ea227ce.drv' failed with exit code 1
error: build of '/nix/store/mh8hl6c7gyyqrnzrln4j2jxg79n32slf-nixpkgs-21.11pre300283.f930ea227ce.drv' failed
error: program '/usr/bin/nix-env' failed with exit code 100
```

Additional info:

nix package version: 2.3.13-3
config: only the effect of running `nix-channel --add https://nixos.org/channels/nixpkgs-unstable` per the wiki
misc: I don't appear to be the first person to encounter this problem. Others workaround this problem by installing nix manually and avoiding the Arch package: https://github.com/NixOS/nix/issues/2673#issuecomment-487773657

Steps to reproduce (following wiki):

1. Install `nix` from the main repo
2. Enable and start `nix-daemon.service`
3. Add your user to the `nix-users` group (and reboot)
4. Run `nix-channel --add https://nixos.org/channels/nixpkgs-unstable`
5. Run `nix-channel --update`