[Issue 21593] Only update file time if file to be written already exists

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21593

--- Comment #5 from Andrei Alexandrescu <andrei at erdani.com> ---
Related: https://issues.dlang.org/show_bug.cgi?id=21597

--