This implements a simple way to upgrade Talos node running in
maintenance mode (only if Talos is installed, i.e. if `STATE` and
`EPHEMERAL` partitions are wiped).
Upgrade is only available over SideroLink for security reasons.
Upgrade in maintenance mode doesn't support any options, and it works
without machine configuration, so proxy environment variables are not
available, registry mirrors can't be used, and extensions are not
installed.
Fixes#6224
Signed-off-by: Andrey Smirnov <andrey.smirnov@talos-systems.com>
