LU-15908 build: fix debian rules

LU-15908 build: fix debian rules

A comment in debian/rules file breaks a commands-line continuation,
causing exported env vars to be dropped.

Lustre-change: https://review.whamcloud.com/47512
Lustre-commit: 81fde14387e0b352ea8211fa74cec37171ea08fb

Fixes: a5084c2f2e ("LU-14937 build: re-use config cache in 'make rpms/debs'")
Test-Parameters: trivial
Signed-off-by: Ake Sandgren <ake.sandgren@hpc2n.umu.se>
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I0894199301c489b1961e043651c74d7c6c0089d2
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/47671
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>