Expose builtins.fetchFinalTree to the user

[edolstra]

Motivation

This may be more efficient than fetchTree since it allows substitution from binary caches if narHash is specified in the input attributes. This is what call-flake.nix already used internally.

Depends on #14633.

Corresponding PR to flake-compat: https://github.com/NixOS/flake-compat/pull/79

Context

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