nix-shell: Overwrite environment variables

Need to remember that std::map::insert() and emplace() don't overwrite
existing entries...

This fixes a regression relative to 1.11 that in particular triggers
in nested nix-shells.

Before:

  $ nativeBuildInputs=/foo nix-shell -p hello --run 'hello'
  build input /foo does not exist

After:

  $ nativeBuildInputs=/foo nix-shell -p hello --run 'hello'
  Hello, world!

src/nix-build/nix-build.cc

@@ -408,7 +408,7 @@ int main(int argc, char ** argv)
                 env["NIX_STORE"] = store->storeDir;
 
                 for (auto & var : drv.env)
-                    env.emplace(var);
+                    env[var.first] = var.second;
 
                 restoreAffinity();