dateMenu: Fix temperature edge case

We currently format the temperature with a precision of 0, that is
with no digits after the decimal-point. As a result, a temperature
like -0.2 shows up as -0.

Math.trunc() has the same effect as `%.0f` and handles that edge
case correctly, but while at it, we can just as well round the
value properly.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/3441

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1548>


(cherry picked from commit e85d127a)