Fix `addTextToStore` for binary caches
Fix `addTextToStore` for binary caches

Because of a too eager refactoring, `addTextToStore` used to throw an
error because the input wasn't a valid nar.

Partially revert that refactoring to wrap the text into a proper nar
(using `dumpString`) to make this method work again

NixOS/nix,NixOS/nix,NixOS/nix,rycee/nix,NixOS/nix,rycee/nix,rycee/nix,rycee/nix,NixOS/nix,rycee/nix