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!