deploy: specifying revision causes series to default to xenial

[Jordan Barrett]

On Juju 2.9.35-latest, if you specify a charm revision, the series defaults to xenial instead of focal:
```
$ juju deploy juju-qa-refresher
Located charm "juju-qa-refresher" in charm-hub, revision 2
Deploying "juju-qa-refresher" from charm-hub charm "juju-qa-refresher", revision 2 in channel stable on focal

$ juju deploy juju-qa-refresher --channel=stable
Located charm "juju-qa-refresher" in charm-hub, revision 2
Deploying "juju-qa-refresher" from charm-hub charm "juju-qa-refresher", revision 2 in channel stable on focal

$ juju deploy juju-qa-refresher --channel=stable --revision=2
Located charm "juju-qa-refresher" in charm-hub, revision 2
Deploying "juju-qa-refresher" from charm-hub charm "juju-qa-refresher", revision 2 in channel stable on xenial
```

IMO, these should all have the same result.

[Juan M. Tirado (tiradojm)]

Does this still occur with 2.9.38?

[Jordan Barrett (barrettj12)]

Still happens on 2.9-latest.

$ juju deploy juju-qa-refresher --channel=stable
Located charm "juju-qa-refresher" in charm-hub, revision 2
Deploying "juju-qa-refresher" from charm-hub charm "juju-qa-refresher", revision 2 in channel stable on jammy

$ juju deploy juju-qa-refresher --channel=stable --revision=2
Located charm "juju-qa-refresher" in charm-hub, revision 2
Deploying "juju-qa-refresher" from charm-hub charm "juju-qa-refresher", revision 2 in channel stable on xenial

$ juju version --all
version: 2.9.40-ubuntu-amd64
git-commit: 934ccf38d3cab2d75143e4cb0b382a113c6fe15d
git-tree-state: clean
compiler: gc

[Jordan Barrett (barrettj12)]

This could also be an issue with the Charmhub API - maybe when we request a certain revision, it returns us xenial by default.