bug#60730: 29.0.60; Free variable with :buffer keyword in ert-with-temp-file

[Eli Zaretskii]

> From: "J.P." <jp@neverwas.me>
> Cc: 60730@debbugs.gnu.org,  stefankangas@gmail.com
> Date: Sat, 28 Jan 2023 18:00:40 -0800
> 
> When the value of `coding-system-for-write' is non-nil, only quoting it
> twice survives expansion:
> 
>   (setq coding-system-for-write ''raw-text)
>   (ert-with-temp-file myfile)
> 
>   -> (let* ((coding-system-for-write 'raw-text) ...)
> 
> Otherwise, we get a free variable:
> 
>   (setq coding-system-for-write 'raw-text)
>   (ert-with-temp-file myfile)
> 
>   -> (let* ((coding-system-for-write raw-text) ...)

OK, in that case we should indeed solve this in the code by something
like the patch you proposed.