Allow usage of default .cols in across()

[markfairbanks]

reprex of issue fixed:

library(dtplyr)
library(dplyr)

test_df <- tibble(x = 1:3, y = 1:3)

lazy_dt(test_df) %>%
  mutate(across(.fns = ~ .x + 1))
#> Error: Can't rename variables in this context.

[hadley]

Thanks!