LU-15596 build: set TARBALL with m-a build helper for debs

LU-15596 build: set TARBALL with m-a build helper for debs

debs built using module-assistant tool by default scan
for packages and may find a random lustre tarball

Specify the correct tarball with the TARBALL
environment variable to avoid default search picking an
incorrect tarball source.

Lustre-change: https://review.whamcloud.com/46639
Lustre-commit: 47b8b6880d93d54ca1d24effb69d8c84d8d0fb2f

HPE-bug-id: LUS-10783
Test-Parameters: trivial
Fixes: 1d97ac16d8 ("LU-14948 build: Warn about /usr/src/lustre.tar.bz2")
Signed-off-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Change-Id: Iaa5a31aaa81e11ee97ae2ea27811c7a4399a0efa
Reviewed-on: https://review.whamcloud.com/47776
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>