Fix cli::format_error call in get_desc_config_flag

[sims1253]

get_desc_config_flag() in R/utils.R reports an invalid DESCRIPTION flag with:

stop(cli::format_error(
  "The {.code {name}} entry in {.path DESCRIPTION} must be {.code TRUE}
   or {.code FALSE}.",
  "i" = "It is {.val {val}}."
))

cli::format_error() has no ..., so the named "i" argument errors with "unused argument" whenever this validation fires, and the user never sees the intended message.

The call now wraps the messages in a named vector, the pattern the sibling format_error() calls in the same file already use:

Error in `get_desc_config_flag()`:
  ! The `Config/build/never-clean` entry in 'DESCRIPTION' must be `TRUE` or `FALSE`.
  i It is "maybe".

Found during my most recent ry audit.