Make `evalcast::get_covidhub_predictions` output a consistent `class`

[brookslogan]

With use_disk=FALSE (default) this function outputs a c("predictions_cards", "data.frame"), while use_disk=TRUE will output a c("predictions_cards", "tbl_df", "tbl", "data.frame") (from collect). It'd be better to have a consistent output class (the latter is probably nicer, but could break some things, e.g., by making all.equal less strict).