ca-derivations downstream placeholder does not start with storeDir

[roberth]

Describe the bug

Do ca-derivations downstream placeholders need to of the format /[a-z0-9]{52}?
The current behavior leads to bugs in code that is not aware of this behavior (e.g. NixOS/nixpkgs#261512), and even when the code is fixed, this placeholder is incomprehensible when it leaks out in e.g. thrown errors in the Nix language.

Steps To Reproduce

Expected behavior

A placeholder contains the storeDir and name, so that relevant info about the placeholder can be understood by code and humans.

If there's a store layer benefit, we could even handle this at the language level, but changing the store layer to work with store path-like placeholders seems like a simpler and more complete solution.

Metadata

Additional context

• minimal install cd.iso can't be built with nixpkgs.config.contentAddressedByDefault = true; nixpkgs#261512
  • lib/types.nix: allow CA paths in pathInStore nixpkgs#377007
• Similar issue: ca-derivations breaks some nix-side store paths manipulations #4764

Checklist

• checked latest Nix manual (source)
• checked open bug issues and pull requests for possible duplicates

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