Improve Windows build process in build.rs

[gugu8intel-i9]

Update the error handling for copying Cargo.toml on Windows.

[mo8it]

I think the error was ignored because it occurs when the file already exists or something like that. I don't see a need to improve it here since you will get an error later anyway.

[senekor]

std::fs::copy will overwrite existing files, so the error must be something else. @gugu8intel-i9 should've said what their actual problem was though.