melodie-switch: `collect_metrics()` broke

[hfrick]

With https://github.com/tidymodels/tune/tree/melodie-switch:

collect_metrics() returns something that looks like a join gone wrong, with NA in the .config column

• tests/testthat/test-engine-parameters-earth.R

> collect_metrics(mars_search)
# A tibble: 22 × 9
   num_terms    nk .metric .estimator  mean     n std_err .config         .iter
       <int> <int> <chr>   <chr>      <dbl> <int>   <dbl> <chr>           <int>
 1         2   200 rmse    standard   3.38      5  0.462  pre0_mod1_post0     0
 2         2   200 rsq     standard   0.717     5  0.0971 pre0_mod1_post0     0
 3         3    20 rmse    standard   2.93      5  0.207  pre0_mod2_post0     0
 4         3    20 rsq     standard   0.828     5  0.0328 pre0_mod2_post0     0
 5         5   110 rmse    standard   2.81      5  0.268  pre0_mod3_post0     0
 6         5   110 rsq     standard   0.851     5  0.0375 pre0_mod3_post0     0
 7         2    20 rmse    standard   3.38      5  0.462  NA                  0
 8         2    20 rsq     standard   0.717     5  0.0971 NA                  0
 9         2   110 rmse    standard   3.38      5  0.462  NA                  0
10         2   110 rsq     standard   0.717     5  0.0971 NA                  0
# ℹ 12 more rows
# ℹ Use `print(n = ...)` to see more rows