Fix issue with update train list

Description

William added support for train redirection. I tested with an internal build using internal update server.

First I created /data/update.conf with this

{
"freenas-11.3-internal": {
"redirect": "FreeNAS-11-Nightlies"
}
}

After running the middlware call below I can confirm FreeNAS-11-Nighlies is the selected train.

midclt call update.get_trains|jq .

{
"trains": {
"FreeNAS-11-Nightlies": {
"description": "Nightly builds for FreeNAS 11. Very unstable, even ISO installs are not guaranteed to work. [nightly]",
"sequence": null
},
"FreeNAS-11-Nightlies-SDK": {
"description": "Nightly SDK builds for FreeNAS 11. Very unstable, even ISO installs are not guaranteed to work. [nightly]",
"sequence": null
},
"FreeNAS-11-STABLE": {
"description": "Release Train for FreeNAS 11.1 [release]",
"sequence": null
},
"FreeNAS-11.2-STABLE": {
"description": "Release Train for FreeNAS 11.2 [release]",
"sequence": null
}
},
"current": "freenas-11.3-internal",
"selected": "FreeNAS-11-Nightlies"
}