address slowdown in fit_xy()

[simonpcouch]

On main dev:

library(parsnip)
bench::mark(
  fit = fit_xy(linear_reg(), mtcars[2:11], mtcars[1])
)
#> # A tibble: 1 × 6
#>   expression      min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr> <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 fit          1.76ms   2.07ms      473.    3.95MB     32.3

With this PR:

library(parsnip)
bench::mark(
  fit = fit_xy(linear_reg(), mtcars[2:11], mtcars[1])
)
#> # A tibble: 1 × 6
#>   expression      min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr> <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 fit          1.27ms   1.48ms      656.    2.79MB     31.7

^{Created on 2024-02-27 with reprex v2.1.0}

[github-actions]

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