R""(
Pin nixpkgs
to its most recent Git revision:
Afterwards the user registry will have an entry like this:
and nix flake metadata
will say:
Pin nixpkgs
in a custom registry to its most recent Git revision:
This command adds an entry to the user registry that maps flake reference url to the corresponding locked flake reference, that is, a flake reference that specifies an exact revision or content hash. This ensures that until this registry entry is removed, all uses of url will resolve to exactly the same flake.
Entries can be removed using nix registry
remove".
)""