Comment 4 for bug 1851040

Upgrade from fails too:

```
$ sudo do-release-upgrade -d
Checking for a new Ubuntu release
Get:1 Upgrade tool signature [1 554 B]
Get:2 Upgrade tool [1 334 kB]
45% [2 606 kB/1 334 kB 45%] Fetched 1 335 kB in 0s (0 B/s)
authenticate 'focal.tar.gz' against 'focal.tar.gz.gpg'
extracting 'focal.tar.gz'

Reading cache

Checking package manager
Reading package lists... Done
Building dependency tree
Reading state information... Done
Hit http://archive.ubuntu.com/ubuntu eoan InRelease
Get:1 http://security.ubuntu.com/ubuntu eoan-security InRelease [92,9 kB]
Hit http://archive.ubuntu.com/ubuntu eoan-updates InRelease
Hit http://archive.ubuntu.com/ubuntu eoan-backports InRelease
Fetched 92,9 kB in 0s (0 B/s)
Reading package lists... Done
Building dependency tree
Reading state information... Done

Updating repository information

No valid sources.list entry found

While scanning your repository information no entry about disco could
be found.

An upgrade might not succeed.

Do you want to continue anyway?

Continue [yN]

Restoring original system state

Aborting
Reading package lists... Done
Building dependency tree
Reading state information... Done
```