packaging: Adapt get_last_modification()

The function is returning "" when called from the script used to cache
the artefacts and one difference noted between this version and the
already working one from the CCv0 is that we make sure to `pushd
${repo_root_dir}` in the CCv0 version.

Let's give it a try here and see if it solves the issue.

Signed-off-by: Fabiano Fidêncio <fabiano.fidencio@intel.com>

tools/packaging/scripts/lib.sh

@@ -106,6 +106,7 @@ get_kata_hash() {
 get_last_modification() {
 	local file="${1}"
 
+	pushd ${repo_root_dir} &> /dev/null
 	# This is a workaround needed for when running this code on Jenkins
 	git config --global --add safe.directory ${repo_root_dir} &> /dev/null
 
@@ -113,6 +114,7 @@ get_last_modification() {
 	[ $(git status --porcelain | grep "${file#${repo_root_dir}/}" | wc -l) -gt 0 ] && dirty="-dirty"
 
 	echo "$(git log -1 --pretty=format:"%H" ${file})${dirty}"
+	popd &> /dev/null
 }
 
 # $1 - The tag to be pushed to the registry