bug#53646: 27.2; auto-insert-mode causes interactive prompt for update-directory-autoloads

[Allen Li]

When `auto-insert-mode` is enabled, `update-directory-autoloads` causes an
interactive prompt to appear for inserting an Emacs Lisp header.

This is because `update-directory-autoloads` calls
`autoload-find-generated-file`, which uses `find-file-noselect`.

Arguably, it's wrong to use `find-file-noselect` programmaticaly
(https://emacs.stackexchange.com/a/2898), but I understand that
`find-file-noselect` does some convenient magic like decoding.  A quick
hacky fix would be to `let`-bind `auto-insert` to nil in
`autoload-find-generated-file`.

In GNU Emacs 27.2 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.27, cairo 
version 1.17.4)
 of 2021-03-26 built on juergen
Windowing system distributor 'The X.Org Foundation', version 11.0.12101003
System Description: Arch Linux