std.debug.SelfInfo.Windows: less invasive change

restores code closer to master branch in hopes of avoiding a regression
that was introduced when this was based on openSelfExe rather than
GetModuleFileNameExW.
6f64c8b693Andrew Kelley committed on 10/29/2025, 1:20:52 PMparent1553c8e
2 files changedLine totals unavailable