title: Authenticating 'focal.tar.gz' against 'focal.tar.gz.gpg' fails
body: <p>When trying to perform a <code>do-release-upgrade</code> to upgrade from Ubuntu 18.04 to 20.04 I am getting the below error</p>
  <pre><code>Checking for a new Ubuntu release
  Get:1 Upgrade tool signature [1,554 B]                                                                                                 
  Get:2 Upgrade tool [1,342 kB]                                                                                                          
  Fetched 1,343 kB in 0s (0 B/s)                                                                                                         
  authenticate 'focal.tar.gz' against 'focal.tar.gz.gpg' 
  Authentication failed
  Authenticating the upgrade failed. 
  There may be a problem with the network or with the server.
  </code></pre>
  <p>FYI: I already tried <code>sudo apt-get install --reinstall ubuntu-keyring</code>.</p>