Maybe you could work around it using run? e.g. something like
function mycp(src::AbstractString, dst::AbstractString; force::Bool=false)
cmd = Sys.iswindows() ? `cmd /c copy` : `cp`
force_option = Sys.iswindows() ? (force ? `/y` : `/-y`) : (force ? `-f` : `-n`)
cmd = `$cmd $force_option`
run(`$cmd $src $dst`)
end