Add missing shebang

Fixes the following shellcheck error:

    SC2148 (error): Tips depend on target shell and yours is unknown. Add a shebang or a 'shell' directive.

https://www.shellcheck.net/wiki/SC2148

hack/env.sh

@@ -1,3 +1,5 @@
+#!/bin/bash
+
 if [ -z $SKIP_VAR_SET ]; then
         export SRIOV_CNI_IMAGE=${SRIOV_CNI_IMAGE:-ghcr.io/k8snetworkplumbingwg/sriov-cni}
         export SRIOV_INFINIBAND_CNI_IMAGE=${SRIOV_INFINIBAND_CNI_IMAGE:-ghcr.io/k8snetworkplumbingwg/ib-sriov-cni}