Avoid eager splicing by using substitute() not enexpr()

[DavisVaughan]

Closes #6

Basically same solution as r-lib/bench#86

This works as expected now

cross::bench_versions(pkgs = c("rlang", "r-lib/rlang#1837"), {
  library(rlang)
  x <- as.list(1:1e6)
  bench::mark(list2(!!!x), iterations = 10)
})
#> # A tibble: 2 × 7
#>   pkg              expression       min   median `itr/sec` mem_alloc `gc/sec`
#>   <chr>            <bch:expr>  <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 rlang            list2(!!!x)    902ns   1.13µs   496738.        0B        0
#> 2 r-lib/rlang#1837 list2(!!!x)    615ns 738.07ns   880486.        0B        0

^{Created on 2025-10-09 with reprex v2.1.1}