Check if NIX_LINK_NEW exists instead of checking that NIX_LINK doesn't exist

[cole-h]

Motivation

For brand new installations, neither NIX_LINK_NEW
($XDG_STATE_HOME/nix/profile or ~/.local/state/nix/profile), nor NIX_LINK (~/.nix-profile) will exist.

This restores functionality to nix-env, which is relied upon by GitHub Actions such as https://github.com/cachix/cachix-action and the Nixpkgs EditorConfig (and other) CI.

Context

Earlier today, Nixpkgs GHA CI started failing because it was relying on nix-env. Although this has since been fixed by switching to nix-shell, there are other places in the ecosystem that rely on 1) the latest Nix version; and 2) nix-env.

See the conversation in #5588 (starting at #5588 (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

---

This might not be the right solution, but it is a solution that fixes the regression. I'm happy to close this in favor of a more comprehensive solution, however.

There's an extra commit pinning the cachix actions to Nix 2.13.3 to see if that will allow CI to pass (it's failing weirdly on master and prior to this change) -- feel free to force-push it away if this overall approach is "good enough" for now.

This should be backported to 2.14.

[cole-h]

I tested this in an Ubuntu 22.04.1 VM using the following (from a now-force-pushed-away commit, but the changes are ~the same):

set -eux
export XDG_STATE_HOME=
curl -L https://cole-h-nix-install-tests.cachix.org/serve/6k97myldi0hn3jq36m323cynafdnsb7z/install > install
chmod +x install
cat /dev/null | ./install --tarball-url-prefix https://cole-h-nix-install-tests.cachix.org/serve --daemon

. /nix/var/nix/profiles/default/etc/profile.d/nix-daemon.sh

nix-env \
-iA editorconfig-checker \
-f '<nixpkgs>' \
-I nixpkgs=flake:nixpkgs \
--extra-experimental-features flakes

echo does-not-exist | xargs -r editorconfig-checker || echo FAILED!!!!!!!!! # if the file doesn't exist, it will print red text but not hit this "FAILED!!!" print; only if the command can't be found will it hit this print
nix-env -e editorconfig-checker

[fricklerhandwerk]

@balsoft @thufschmitt

[balsoft]

I'm generally in favor of a fix, but I think there's a better solution. I'll try working on it separately

[nixos-discourse]

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

https://discourse.nixos.org/t/nix-2-14-0-released/25900/7

[balsoft]

My solution is here: #7929

It's probably not perfect, but should be more correct in theory.

I'm still in favor of merging this PR first and rolling out the hotfix, since my solution will probably take longer to review and merge.

[Ericson2314]

Agreed with @balsoft's plan to move this first. Beyond the reasons given, when we make this feature non-experimental but still don't to immediately migrate all existing users, this will be the logic we want.

[github-actions]

Successfully created backport PR for 2.14-maintenance:

• [Backport 2.14-maintenance] Check if NIX_LINK_NEW exists instead of checking that NIX_LINK doesn't exist #7934

[github-actions]

Git push to origin failed for 2.14-maintenance with exitcode 1

[cole-h]

(FYI, this still included the commit pinning the install-nix-action to 2.13.3 -- I don't know if we want to keep that around, especially since v20 of the action fixes the incompatibility with 2.14.0)

[roberth]

This was merged without a test case. I hope that's not a pattern.