build/build-release.sh: sync Go version with go.mod file
build/build-release.sh: sync Go version with go.mod file

We have Go 1.14 available already, but go.mod still mentions 1.13, so we
should at least update to latest release of it.

Signed-off-by: Mateusz Gozdek <c74780982fca069d559e5c78b4677476dc80851c@kinvolk.io>

kubernetes-incubator/bootkube,kubernetes-incubator/bootkube,kubernetes-incubator/bootkube