tweak how default sysimage is replaced on Windows

[KristofferC]

No description provided.

[rapus95]

I'd rather replace it with something like that:

function prepareto_writesysimg()
    if Sys.iswindows() && isfile(default_sysimg_path())
        mv(default_sysimg_path(), tempname())
    end
end

to prevent the process to abort if there's nothing to overwrite 😁 (Also it extracts that special behaviour into a function 👼)

[KristofferC]

Done