pkg/invoke: ensure that custom env vars are prepended to the env
pkg/invoke: ensure that custom env vars are prepended to the env

Ensure that custom values take priority over values defined in the
environment by prepending them to the env, not appending.

dcbw/cni,dcbw/cni,containernetworking/cni,kad/cni,rosenhouse/cni,containernetworking/cni,rosenhouse/cni,appc/cni,appc/cni,kad/cni