nix: Add --flake flag

This allows using an arbitrary "provides" attribute from the specified
flake. For example:

  nix build --flake nixpkgs packages.hello

(Maybe provides.packages should be used for consistency...)

src/libexpr/eval.hh

@@ -318,8 +318,6 @@ public:
 
     const FlakeRegistry & getFlakeRegistry();
 
-    Value * makeFlakeRegistryValue();
-
 private:
     std::unique_ptr<FlakeRegistry> _flakeRegistry;
     std::once_flag _flakeRegistryInit;