Redirecting an entire site except for the certbot webroot

In order to be able to use the webroot plugin for certbot and automatically renew the Let's Encrypt certificate for libravatar.org, I had to put together an Apache config that would do the following on port 80:

Let /.well-known/acme-challenge/* through on the bare domain (http://libravatar.org/).
Redirect anything else to https://www.libravatar.org/.

The reason for this is that the main Libravatar service listens on www.libravatar.org and not libravatar.org, but that cerbot needs to ascertain control of the bare domain.

This is the configuration I ended up with:

<VirtualHost *:80>
    DocumentRoot /var/www/acme
    <Directory /var/www/acme>
        Options -Indexes
    </Directory>

    RewriteEngine on
    RewriteCond "/var/www/acme%{REQUEST_URI}" !-f
    RewriteRule ^(.*)$ https://www.libravatar.org/ [last,redirect=301]
</VirtualHost>

The trick I used here is to make the redirection RewriteRule conditional on the requested file (%{REQUEST_URI}) not existing in the /var/www/acme directory, the one where I tell certbot to drop its temporary files.

Here are the relevant portions of /etc/letsencrypt/renewal/www.libravatar.org.conf:

[renewalparams]
authenticator = webroot
account = 

[[webroot_map]]
libravatar.org = /var/www/acme
www.libravatar.org = /var/www/acme