'juju download' does NOT respect non-latest default track

[Alex Lutay]

Hi,

The default track has been switched for some charms,
e.g: mysql/mysql-k8s/postgresql/postgresql-k8s/...

Juju respect non-latest default track everywhere, but not in `juju download`:

> juju deploy mysql
Located charm "mysql" in charm-hub, revision 151
Deploying "mysql" from charm-hub charm "mysql", revision 151 in channel 8.0/stable on jammy

> juju download mysql
ERROR "mysql" has no releases in channel "stable". Type
    juju info mysql
for a list of supported channels.

> juju download mysql --channel 8.0/stable
Series "focal" is not supported for charm "mysql", trying series "jammy"
Fetching charm "mysql" using "8.0/stable" channel and base "amd64/ubuntu/22.04"
Install the "mysql" charm with:
    juju deploy ./mysql_151716c.charm

It is a ticket to address this issue on Juju.
Expected results: `juju download mysql` fetches charm properly from default 8.0/stable track.