LU-16784 tests: fix path to lgss_sk

LU-16784 tests: fix path to lgss_sk

Find correct path to lgss_sk utility, by looking inside Lustre build
tree if command is not installed on the local node.

Test-Parameters: trivial
Test-Parameters: mdscount=2 mdtcount=4 osscount=1 ostcount=8 clientcount=2 testlist=sanity-sec env=SHARED_KEY=true
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I23920bb2a44d2ec7e9662e75c23bd5302d8dfee2
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50825
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Sarah Liu <sarah@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>