wolfictl adv guide: support existing clone of advisories repo

[luhring]

The guide command was designed to solve for the issue of wolfictl adv (create|update) having very specific expectations for a user to have already cloned the relevant advisories repo and pulled the latest changes fairly recently. This was fairly harmful to users, who didn't all have the same exact setup locally.

But it's still possible for users to want the ease of the guide command's advisory generation logic but want to use their own non-temporary clone of the advisories repo. We should allow the user to pass in a path to an existing advisories repo, similar to how other wolfictl adv ... commands work, when invoking the wolfictl adv guide command.