Propagate DEBUG variable to the debuild (used also for running run_tests.sh)

Change-Id: I54538795f5ac592f3d58afd865b99d9f9807ea70
diff --git a/src/com/mirantis/mk/Debian.groovy b/src/com/mirantis/mk/Debian.groovy
index f31b762..2d75d1e 100644
--- a/src/com/mirantis/mk/Debian.groovy
+++ b/src/com/mirantis/mk/Debian.groovy
@@ -50,7 +50,7 @@
             [ ! -f pre_build_script.sh ] || bash ./pre_build_script.sh &&
             sudo -H -E -u jenkins dpkg-source -x ${file} build-area/${pkg} && cd build-area/${pkg} &&
             mk-build-deps -t "apt-get -o Debug::pkgProblemResolver=yes -y" -i debian/control &&
-            sudo -H -E -u jenkins debuild --no-lintian -uc -us -b'""")
+            sudo -H -E -u jenkins debuild --preserve-envvar DEBUG --no-lintian -uc -us -b'""")
     }