OK, how about this:
<VirtualHost *:80>
ServerAdmin webmaster@localhost
DocumentRoot /var/www/html
# Redirect to HTTPS, except for ACME challenge
RewriteEngine On
RewriteCond %{REQUEST_URI} !^/\.well\-known/acme\-challenge/
RewriteRule ^(.*)$ https://%{HTTP_HOST}$1 [R=301,L]
</VirtualHost>