Installation fails on ubuntu server 20.04 LTS

/label funkwhale65 funkwhale81

Steps to reproduce

1. Run sudo sh -c "$(curl -sSL https://get.funkwhale.audio/)"
2. Skip nginx and certbot installation
3. Observe installation failure

What happens?

TASK [funkwhale : Install system packages] ***********************************************************************************************************************************************************************

 [WARNING]: Could not find aptitude. Using apt-get instead

fatal: [127.0.0.1]: FAILED! => {"changed": false, "msg": "No package matching 'python-virtualenv' is available"}

What is expected?

Successful installation.

Context

Funkwhale version(s) affected: 0.21.1

$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 20.04 LTS
Release:        20.04
Codename:       focal