sdk: make downloads from UpdateFile acknowledge umask
sdk: make downloads from UpdateFile acknowledge umask

This drops the use of ioutil.TempFile in favor of a predetermined
temporary file name.

marineam/mantle,quantum/coreos-mantle,quantum/coreos-mantle,dm0-/mantle,marineam/mantle,glevand/coreos--mantle,dm0-/mantle,glevand/coreos--mantle