Stabilize `nix copy`

[bjornfor]

Describe the bug

Starting with Nix 2.x, nix-push got removed and replaced with nix copy. But later, nix copy was marked as an experimental command, meaning there is no longer a stable CLI to copy paths around?

Expected behavior

When removing a stable command, there is a migration path to a new stable command. (Preferably with some overlap in time.)

Priorities

Add 👍 to issues you find important.

[bjornfor]

Related: #7701

[fricklerhandwerk]

Discussed in the Nix team meeting 2023-03-17:

We'll try to stabilize nix copy (with a very small subset of installable and flags so that we don't commit too much).

This is now part of #7701 (comment)

[nixos-discourse]

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

https://discourse.nixos.org/t/2023-03-17-nix-team-meeting-minutes-41/26614/1

[fricklerhandwerk]

Discussed in Nix team meeting 2023-03-20:

We still agree we'll want to make that functionality stable. But we had a long discussion on the general design approach, without conclusion for now.

[nixos-discourse]

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

https://discourse.nixos.org/t/2023-03-20-nix-team-meeting-minutes-42/26615/1