address slowdown in get_model_spec()

[simonpcouch]

This env_obj isn't used elsewhere in the function and accounts for a decent chunk of the overhead in this example.

With dev parsnip:

library(parsnip)

bench::mark(fit = fit(linear_reg(), mpg ~ ., mtcars))
#> # 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          1.51ms   1.78ms      547.    3.76MB     32.5

With this PR:

library(parsnip)

bench::mark(fit = fit(linear_reg(), mpg ~ ., mtcars))
#> # 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          1.41ms   1.51ms      642.    3.75MB     36.7

^{Created on 2024-02-26 with reprex v2.1.0}

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