Inhibit flymake lexical cookie warning in *scratch* (bug#70068)

Inhibit flymake lexical cookie warning in *scratch* (bug#70068)

Add a hack to prevent flymake from complaining about a missing lexical
cookie when enabled in *scratch*.

* lisp/emacs-lisp/bytecomp.el
(bytecomp--inhibit-lexical-cookie-warning): New variable.
(byte-compile-file): Use it to gate the warning.
* lisp/progmodes/elisp-mode.el (elisp-flymake-byte-compile):
Set the inhibitor when spawning a check in lisp-interaction-mode.

(cherry picked from commit cda5155c618d774810ce5e044efe1101846054ac)