coercing a datetime to a datetime loses timezone

a `POSIX*t` object's timezone is lost when it is coerced to a `POSIX*t` object using `as_datetime()`:

```r
x = as.POSIXct("2022-01-01 23:40:36", tz = "US/Pacific")
as_datetime(x)
## [1] "2022-01-02 07:40:36 UTC"
identical(x, as_datetime(x))
## [1] FALSE

# not the case with as.POSIXct
identical(x, as.POSIXct(x))
## [1] TRUE

```