Comment 0 for bug 1964115

I'm getting the following error since I've updated the installer ISO image from the Ubuntu 20.04.3 LTS to Ubuntu 20.04.4 LTS:
```
Mar 08 11:08:13 ubuntu-server subiquity_event.1858[1858]: subiquity/ErrorReporter/1646737692.998598814.unknown/add_info: written to /var/crash/1646737692.998598814.unknown.crash
Mar 08 11:08:13 ubuntu-server subiquity_event.1858[1858]: subiquity/Mirror/cmd-apt-config: curtin command apt-config
Mar 08 11:08:12 ubuntu-server subiquity_event.1858[1858]: subiquity/ErrorReporter/1646737692.998598814.unknown/add_info
Mar 08 11:08:12 ubuntu-server subiquity_log.1858[2141]: 'RELEASE'
Mar 08 11:08:12 ubuntu-server subiquity_log.1858[2141]: KeyError: 'RELEASE'
Mar 08 11:08:12 ubuntu-server subiquity_log.1858[2141]: return str(selected_params[key])
Mar 08 11:08:12 ubuntu-server subiquity_log.1858[2141]: File "/snap/subiquity/3119/lib/python3.8/site-packages/curtin/util.py", line 1191, in replacer
Mar 08 11:08:12 ubuntu-server subiquity_log.1858[2141]: return BASIC_MATCHER.sub(replacer, content)
Mar 08 11:08:12 ubuntu-server subiquity_log.1858[2141]: File "/snap/subiquity/3119/lib/python3.8/site-packages/curtin/util.py", line 1193, in basic_template_render
Mar 08 11:08:12 ubuntu-server subiquity_log.1858[2141]: return basic_template_render(content, params)
Mar 08 11:08:12 ubuntu-server subiquity_log.1858[2141]: File "/snap/subiquity/3119/lib/python3.8/site-packages/curtin/util.py", line 1203, in render_string
Mar 08 11:08:12 ubuntu-server subiquity_log.1858[2141]: entry.uri = util.render_string(entry.uri, mirrors)
Mar 08 11:08:12 ubuntu-server subiquity_log.1858[2141]: File "/snap/subiquity/3119/lib/python3.8/site-packages/curtin/commands/apt_config.py", line 255, in update_mirrors
Mar 08 11:08:12 ubuntu-server subiquity_log.1858[2141]: entries = update_mirrors(entries, mirrors)
Mar 08 11:08:12 ubuntu-server subiquity_log.1858[2141]: File "/snap/subiquity/3119/lib/python3.8/site-packages/curtin/commands/apt_config.py", line 354, in generate_sources_list
Mar 08 11:08:12 ubuntu-server subiquity_log.1858[2141]: generate_sources_list(cfg, release, mirrors, target)
Mar 08 11:08:12 ubuntu-server subiquity_log.1858[2141]: File "/snap/subiquity/3119/lib/python3.8/site-packages/curtin/commands/apt_config.py", line 77, in handle_apt
Mar 08 11:08:12 ubuntu-server subiquity_log.1858[2141]: handle_apt(apt_cfg, target)
Mar 08 11:08:12 ubuntu-server subiquity_log.1858[2141]: File "/snap/subiquity/3119/lib/python3.8/site-packages/curtin/commands/apt_config.py", line 663, in apt_command
Mar 08 11:08:12 ubuntu-server subiquity_log.1858[2141]: ret = args.func(args)
Mar 08 11:08:12 ubuntu-server subiquity_log.1858[2141]: File "/snap/subiquity/3119/lib/python3.8/site-packages/curtin/commands/main.py", line 202, in main
Mar 08 11:08:12 ubuntu-server subiquity_log.1858[2141]: Traceback (most recent call last):
Mar 08 11:08:12 ubuntu-server curtin_event.1858.1[2141]: finish: cmd-apt-config: FAIL: curtin command apt-config
Mar 08 11:08:12 ubuntu-server subiquity_log.1858[2141]: finish: cmd-apt-config: FAIL: curtin command apt-config
Mar 08 11:08:12 ubuntu-server systemd[1]: tmp-tmpzgi4knbz-mount-dev.mount: Succeeded.
```

I'm using Packer (https://www.packer.io/) to perform an automated installation so nothing else has changed (and if I downgrade to Ubuntu 20.04.3 LTS everything works again).

The relevant part of my Ubuntu autoinstall configuration:
```
  apt:
    primary:
      - arches: [default]
        uri: "http://ubuntu.my-company.tld"
    sources_list: | # written by curtin custom template
      deb $MIRROR/$RELEASE $RELEASE main restricted
      deb $MIRROR/$RELEASE-updates $RELEASE-updates main restricted
      deb $MIRROR/$RELEASE-security $RELEASE-security main restricted

      deb $MIRROR/$RELEASE $RELEASE universe
      deb $MIRROR/$RELEASE-updates $RELEASE-updates universe
      deb $MIRROR/$RELEASE-security $RELEASE-security universe
```

I've based that on the following example configuration:
https://github.com/canonical/curtin/blob/600d6a19e26d773d1c4efae874e8c8785d0e3196/examples/apt-source.yaml#L118