Screenshots use `%Y-%m-%d %H-%M-%S` for the timestamp, which has the
advantage of allowing proper lexicographical sorting.
The screencast file name pattern, on the other hand, uses
locale-dependent expansions, which break sorting based on file name, and
introduces the chance of potentially invalid characters on different
file systems.
Fixes: #5115
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2300>