issue with `tunable` entries in `tune_args.model_spec`

[simonpcouch]

e.g.:

library(tidymodels)

tune_args(
  # known tunable
  linear_reg(penalty = tune()) %>%
    # not known tunable
    set_engine("glmnet", dfmax = tune())
)
#> # A tibble: 2 × 6
#>   name    tunable id      source     component  component_id
#>   <chr>   <lgl>   <chr>   <chr>      <chr>      <chr>       
#> 1 penalty TRUE    penalty model_spec linear_reg <NA>        
#> 2 dfmax   TRUE    dfmax   model_spec linear_reg <NA>

^{Created on 2024-04-03 with reprex v2.1.0}

[simonpcouch]

Related to #1104.