Stabilise `nix key`

[fricklerhandwerk]

This implements NixOS/rfcs#136. This issue is agreed-upon by the https://github.com/orgs/NixOS/teams/nix-team

• Make sure global flags are used consistently
• Otherwise stabilise as is
• Would be great if we had some default behavior for setting up keys and putting them into standard location
  • Have to figure out the details as some may have to be system-wide, some are specific to a store, etc.

[nixos-discourse]

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

https://discourse.nixos.org/t/2023-08-28-nix-team-meeting-minutes-83/32290/1

[thufschmitt]

Closing since this doesn't provide much value.
We already track in #7701 that nix key is in the stabilisation batch, that's enough

[fricklerhandwerk]

I think it's fine to keep since it's already linked in the tracking issue.

[cole-h]

According to the stabilisation issue, nix key is done (has been checked off). Does that mean this issue can be closed?

[cole-h]

I'm going to take the silence as a yes. If there's more to be done for preparing nix key for stabilization, let's un-tick the box in #7701 and reopen this issue with information on what, exactly, is missing.