emacs: enable lexical-binding in some files to fix warnings

Emacs 31 introduces warnings like this:

  Warning (files): Missing `lexical-binding' cookie in "/nix/store/hash-elpa2nix.el".
  You can add one with `M-x elisp-enable-lexical-binding RET'.

pkgs/applications/editors/emacs/build-support/elpa2nix.el

@@ -1,3 +1,5 @@
+;; -*- lexical-binding: t; -*-
+
 (require 'package)
 (package-initialize)
 

pkgs/applications/editors/emacs/build-support/melpa2nix.el

@@ -1,3 +1,5 @@
+;; -*- lexical-binding: t; -*-
+
 (require 'package-recipe)
 (require 'package-build)
 

pkgs/applications/editors/emacs/build-support/mk-wrapper-subdirs.el

@@ -1,3 +1,5 @@
+;; -*- lexical-binding: t; -*-
+
 (defmacro mk-subdirs-expr (path)
   `(setq load-path
          (delete-dups (append '(,path)