MacOS fresh install - default channel and nix upgrade

[patryk-kozak]

Similar scenario to #51

After fresh install of nix on MacOS the nix-info -m output is

- system: `"x86_64-darwin"`
 - host os: `Darwin 21.5.0, macOS 10.16`
 - multi-user?: `yes`
 - sandbox: `no`
 - version: `nix-env (Nix) 2.10.3`
 - channels($USER): `""`
 - channels(root): `"nixpkgs"`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixpkgs`

Questions:

• should channels for $USER be set vs root (multi-user install so probably root is OK)
• should nixpkgs point to per-user profile for root or $USER?
• when using nix-env it fails as it seems to be installed via nix profile ?
• why nix upgrade-nix is failing? Is it because of nix-env failure?

 -> nix-channel --update && nix upgrade-nix
unpacking channels...
replacing old 'nix-2.10.3'
installing 'nix-2.10.3'
building '/nix/store/1bf2kfdwficnlwq679rby2013i6q8c89-user-environment.drv'...
error: opening lock file '/nix/var/nix/profiles/default.lock': Permission denied
error: program '/nix/store/3b7qrm0qjw57fmznrsvm0ai568i89hc2-nix-2.10.3/bin/nix-env' failed with exit code 1

Suggested to run nix profile install nixpkgs#bash -vvvv --extra-experimental-features nix-command --extra-experimental-features flakes to check if it errors, but that's actually passing.

@Helcaraxan