EX-5975 build: check OS type before using dpkg

EX-5975 build: check OS type before using dpkg

Bright cluster manager by default installs dpkg
on it's centos/rhel installation - presumably to
allow provisioning debian nodes in the cluster,
so dpkg is in the path and can't be removed.

This patch fixes LB_USES_DPKG to check OS type
before checking if dpkg is installed.

Test-Parameters: trivial clientdistro=el8.6
Test-Parameters: trivial clientdistro=ubuntu2204 env=SANITY_EXCEPT="130 244a"

Change-Id: Idc9f6edc91f9c89b40f259421b088287e08bfe9c
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/48616
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Gaurang Tapase <gtapase@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>