Fix registry pin ref lookup

[pkpbynum]

Motivation

Closes #14342

The ref was being removed before the lockedRef.resolve lookup. This happened whenever the url was previously in the registry returned by getRegistry(). The user registry errs by default, but you get the same error by specifying --registry:

$ nix registry add foo github:NixOS/nixpkgs/master --registry ./custom.json
$ nix registry pin foo --registry ./custom.json
error: cannot find flake 'flake:foo' in the flake registries

Context

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