environment: Always honor easing duration settings

environment: Always honor easing duration settings

In case an actor easing is created without an explicit duration, we
should rely to the default animation that Clutter.Actor.save_easing_state()
sets to 250ms.

This fine in normal use cases, but in case the animations are disabled
the current code skips calling our Clutter.Actor.set_easing_duration()
override, and so we just end up leaving the actor easing time to 250ms,
not respecting the St settings

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3983>