CI: use split target and subtarget in label workflow

CI: use split target and subtarget in label workflow

With eecc6e48117b ("CI: rework build workflow to have split target and
subtarget directly") target and subtarget are split in 2 different
variables. Label workflow were not aligned to this change and are
currently broken.

Fix them and correctly pass split target and subtarget.

Fixes: eecc6e48117b ("CI: rework build workflow to have split target and subtarget directly")
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
(cherry picked from commit bf8187d5dc4d4bbb23770955744bca1787f32ac0)