Is your feature request related to a problem? Please describe.

The storePath builtin is disallowed in pure mode. This hinders a use case for binary-only distribution via binary caches.

Describe the solution you'd like

storePath isn't impure. It's not even non-hermetic, because it specifies that a store path should exist, which is the same level of hermeticity that a fetcher provides (both built-in or FOD); for example that a git commit should exist.

Describe alternatives you've considered

Use Docker to distribute binary-only software that was originally built with Nix.