nix upgrade-nix: Give a better error message if the profile is using 'nix profile' (backport #12166)

[mergify]

Motivation

DeterminateSystems/nix-installer#1362

Before:

error: directory '/nix/var/nix/profiles/per-user/eelco/test/bin' does not appear to be part of a Nix profile

After:

error: directory "/nix/var/nix/profiles/per-user/eelco/test" is managed by 'nix profile' and currently cannot be upgraded by 'nix upgrade-nix'

Context

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This is an automatic backport of pull request #12166 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of fccfdbe has failed:

On branch mergify/bp/2.24-maintenance/pr-12166
Your branch is up to date with 'origin/2.24-maintenance'.

You are currently cherry-picking commit fccfdbea5.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   src/nix/upgrade-nix.cc

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally