JSON: print paths as strings without copying them to the store

Makes `printValueAsJSON` not copy paths to the store for `nix eval
--json`, `nix-instantiate --eval --json` and `nix-env --json`.

Fixes https://github.com/NixOS/nix/issues/5612

src/libexpr/value.hh

@@ -99,7 +99,7 @@ class ExternalValueBase
 
     /* Print the value as JSON. Defaults to unconvertable, i.e. throws an error */
     virtual void printValueAsJSON(EvalState & state, bool strict,
-        JSONPlaceholder & out, PathSet & context) const;
+        JSONPlaceholder & out, PathSet & context, bool copyToStore = true) const;
 
     /* Print the value as XML. Defaults to unevaluated */
     virtual void printValueAsXML(EvalState & state, bool strict, bool location,