Add use of GIT_DEPTH

Status Job ID Name Coverage
  Build
passed #20460
check-syntax

00:00:48

failed #20461
check-unknown-packages

00:00:43

 
Name Stage Failure
failed
check-unknown-packages Build
done.
$ ./check-external/unknown-packages.py
Traceback (most recent call last):
File "./check-external/unknown-packages.py", line 26, in <module>
import security_db
File "/builds/carnil/test-sec-tracker/lib/python/security_db.py", line 29, in <module>
from apt_pkg import version_compare
ImportError: No module named apt_pkg
ERROR: Job failed: exit code 1