[Bug 2052549] [NEW] 18.04 fails with "Unknown error" when attempting to update to the latest version of the installer.

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When using the 18.04.06 Ubuntu server installer, the installation
process will prompt the user to update the installer to the latest
version; The "Installer update available" step. This is because the
18.04.06 installer runs version 21.08.02 of the installer. However this
update process fails with an "Unknown error".

The reason for this error seems to be that the 18.04.06 installer runs
an older version of `snapd` that does not have the features required for
the newer installer to work. More specifically the latest version of the
installer requires the latest version of `subiquity`, which in-turn
requires the core22 snap. The latest version of `subiquity` requires
snapd >= version 2.55.5. This is reflected in the messages seen during
an attempted update of the installer:

```
Consider re-refresh of "subiquity"
Download snap "core22" (1033) from channel "stable"
```

Testing shows that it is either extremely difficult or impossible to
update the installation environment to use the appropriate version of
`snapd` without a restart of the instance; Which defeats the purpose of
the live installation.

A tested workaround is to use the installer that comes with the 18.04.06
ISO without updating, as the installation works without issue using the
included installer.

** Affects: subiquity (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: bot-comment
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18.04 fails with "Unknown error" when attempting to update to the latest version of the installer.
https://bugs.launchpad.net/bugs/2052549
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