screenshot: Fix stripping suffix from relative filename

screenshot: Fix stripping suffix from relative filename

String.prototype.substr() doesn't support a negative length value
(to subtract from the full length), so we end up with a filename
of '' when hitting that code paths (a relative filename with '.png'
suffix).

Fix this by switching to String.prototype.replace() instead.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/2091