nix: Enable pure mode by default

We want to encourage a brave new world of hermetic evaluation for
source-level reproducibility, so flakes should not poke around in the
filesystem outside of their explicit dependencies.

Note that the default installation source remains impure in that it
can refer to mutable flakes, so "nix build nixpkgs.hello" still works
(and fetches the latest nixpkgs, unless it has been pinned by the
user).

A problem with pure evaluation is that builtins.currentSystem is
unavailable. For the moment, I've hard-coded "x86_64-linux" in the
nixpkgs flake. Eventually, "system" should be a flake function
argument.

src/libexpr/eval.hh

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