Actually ignore system/user registries during locking (2nd attempt)

[edolstra]

Motivation

Makes #12068 actually work. All registries are used for resolving top-level flakerefs, but only the global and flag registries are used for resolving flake inputs.

Fixes #13050.

Context

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[SharzyL]

Should we also add tests to verify user registry not ignored with other operations (to prevent something like #13050)?

[Mic92]

@edolstra it looks like the test was not able to capture the previous regression. Do you know why this is the case?

[Mic92]

Ah, We actually tried to use a flake from the registry. So this usecase is not tested in this pull request.

[Mic92]

Should we also add tests to verify user registry not ignored with other operations (to prevent something like #13050)?

Yeah. At least in nixpkgs we have now a patch merged and backported, so we can take this pull request a bit slower.

[Mic92]

Looks like the test is only testing "global" registry entries:

\[\][\[\]joerg@turingmachine:/home/joerg/git/nix/tests/functional/flakes]$\[\] nix registry list
global flake:flake2 git+file:///tmp/nix-test/flakes/flakes/flake%202
global flake:flake1 git+file:///tmp/nix-test/flakes/flakes/flake1
global flake:flake3 git+file:///tmp/nix-test/flakes/flakes/flake%2520
global flake:nixpkgs flake:flake1

[Mic92]

I added a regression test and made sure it fails before your fix: 57e3906
@edolstra would you mind pushing branches to the Nix repository directly. This would simplify collaboration because I am not allowed to push DeterminateSystems