* Eliminate all uses of the global variable ‘store’ from libstore.

This should also fix:

    nix-instantiate: ./../boost/shared_ptr.hpp:254: T* boost::shared_ptr<T>::operator->() const [with T = nix::StoreAPI]: Assertion `px != 0' failed.

  which was caused by hashDerivationModulo() calling the ‘store’
  object (during store upgrades) before openStore() assigned it.

src/nix-instantiate/nix-instantiate.cc

@@ -64,9 +64,8 @@ void processExpr(EvalState & state, const Strings & attrPaths,
                     if (gcRoot == "")
                         printGCWarning();
                     else
-                        drvPath = addPermRoot(drvPath,
-                            makeRootName(gcRoot, rootNr),
-                            indirectRoot);
+                        drvPath = addPermRoot(*store, drvPath,
+                            makeRootName(gcRoot, rootNr), indirectRoot);
                     std::cout << format("%1%\n") % drvPath;
                 }
             }