Nix: Multi-user install: allow /nix to exist

[jbgi]

Currently multi user installer requires that /nix is absent.
However there are many case where one would want to /nix to be a mount point, so the installer should just check/ensure it has correct access rights (and modify them if needed).

Cleanup instruction also need to be updated accordingly.

[edolstra]

The real issue here is that the installer should be idempotent, i.e. it should allow /nix to exist.

[fiksn]

It would be great if it was possible to run the installer on top of any state in a truly idempotent way. It is really frustrating that first time I forgot --daemon and then I had to manually edit bashrc, zshrc & co just to be able to run it again. Sure better safe than sorry, but perhaps for start there could just be a 'force' flag to warn and not stop with an error.

[abathur]

This can be closed; the core issue was fixed in #3054.

[abathur]

For the record, I noticed after commenting on this one that despite #3054 getting merged, the commit that fixed this issue hasn't made it into 2.3-maintenance.

So, this is "fixed", but you'll still see this on current installers until that is either cherry picked or there's a new release from master.