nix registry pin: Mark the resulting registry entry as exact

[edolstra]

Motivation

Since the resulting target flakeref contains attributes like lastModified or narHash, it's not suitable for overriding. This led to errors like

$ nix build nixpkgs/24.05#hello
error: 'lastModified' attribute mismatch in input 'github:NixOS/nixpkgs/63dacb46bf939521bdc93981b4cbb7ecb58427a0', expected 1728538411, got 1717179513

after doing nix registry pin nixpkgs.

Context

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[roberth]

What is exact? It's not documented in the manual or on the C++ field.

[edolstra]

It means that the registry entry is only used to rewrite the given flakeref if it's exactly equal to the from of the entry. This is useful if you have a registry entry like nixpkgs -> path:/nix/store/... that should match nixpkgs but not e.g. nixpkgs/<branch>. See https://github.com/NixOS/nixpkgs/blob/22d52eb36183414bc4e0dc17ed19955c4988d931/nixos/modules/config/nix-flakes.nix#L68-L76.

[Mic92]

Added a test in https://github.com/NixOS/nix/pull/13229