scripts: resolve shellcheck warnings

Export vpp_binary and vppctl_binary variables before being referenced.

Signed-off-by: Yefeng Lin <yefeng.lin@arm.com>
Reviewed-by: Tianyu Li <tianyu.li@arm.com>
Reviewed-by: Liangxing Wang <liangxing.wang@arm.com>

Change-Id: Ie90f3aed87e4589e82a4903648e81e895f482598