Ok, I got an Ubuntu 20.04 and this worked... well, I mean, I installed the plugin without nginx as dependency but I've not checked that the plugin is working fine.
1.- Download the package python3-certbot-nginx:
apt-get download python3-certbot-nginx
2.- Install several dependencies needed by the plugin:
sudo apt-get install certbot python3-acme python3-certbot python3-mock
3.- Now use dpkg to install the plugin we have download without nginx as dependency (check that the name of the deb file is the right one in your system):
sudo dpkg -i --ignore-depends=nginx python3-certbot-nginx_0.40.0-0ubuntu0.1_all.deb
Cheers,
sahsanu