LU-17850 build: prefer LINUXRELEASE over uname -r

LU-17850 build: prefer LINUXRELEASE over uname -r

In a container or chroot environment, "uname -r" reports
the host instead of the target kernel version. We should
use the LINUXRELEASE variable which is configured in
config/lustre-build-linux.m4 with the value from UTS_RELEASE.

Change-Id: Iaa48027f5ae873e1298695a264db1c351d9eac5c
Test-Parameters: trivial
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55108
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: ake sandgren <ake.sandgren@hpc2n.umu.se>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>