Fix nix-collect-garbage -d with the new profile location

[thufschmitt]

Low-hanging fix for #5226 (comment)

Checklist for maintainers

Maintainers: tick if completed or explain if not relevant

• agreed on idea
• agreed on implementation strategy
• tests, as appropriate
  • functional tests - tests/**.sh
  • unit tests - src/*/tests
  • integration tests - tests/nixos/*
• documentation in the manual
• code and comments are self-explanatory
• commit message explains why the change was made
• new feature or incompatible change: updated release notes

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[Ericson2314]

LGTM, and I assume we'll want the backport too?

[github-actions]

Successfully created backport PR for 2.14-maintenance:

• [Backport 2.14-maintenance] Fix nix-collect-garbage -d with the new profile location #8001

[rycee]

I think it may be worth keeping removal of old generations also for profiles in the old Nix state directory, if it exists. Otherwise, this changed behavior could cause unexpected accumulation of garbage for people who use custom profiles in the per-user directory. And the documentation for nix-collect-garbage still specifically say that it will look in the old directory:

However, it provides two additional options: -d (--delete-old), which deletes all old generations of all profiles in /nix/var/nix/profiles by invoking nix-env --delete-generations old on all profiles (of course, this makes rollbacks to previous configurations impossible); and --delete-older-than period, where period is a value such as 30d, which deletes all generations older than the specified number of days in all profiles in /nix/var/nix/profiles (except for the generations that were active at that point in time).

[Ericson2314]

@rycee See tests/user-envs-migration.sh. I have misunderstood the intent before, and do not yet understand how it actually works, but the intent seems to be to migrate by movi9ng everything to the new directory so it can be garbage collected there.

I agreed these references to the old directory should be cleaned up, however.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2023-03-10-nix-team-meeting-minutes-39/26279/1