fix: handle missing BUILD_DEPENDENCIES in diff output

The pipefail option causes the script to exit if grep finds no matches.
Allow the pipeline to fail gracefully so BUILD_DEPENDENCIES can be empty
if there are no new packages installed.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Author: amannocci

src/usr/src/install-ignity.sh

@@ -178,7 +178,7 @@ pkg::install "s6-portable-utils" "${S6_PORTABLE_UTILS_VERSION}" "${S6_PORTABLE_U
 
 # Diff to do iso uninstall
 dpkg -l | awk '{print $2;}' > /tmp/after
-BUILD_DEPENDENCIES=$(diff /tmp/before /tmp/after | grep ">" | awk '{print $2;}')
+BUILD_DEPENDENCIES=$(diff /tmp/before /tmp/after | grep ">" | awk '{print $2;}' || true)
 
 # Purge build dependencies and cleanup apt
 apt-get purge -y --auto-remove ${BUILD_DEPENDENCIES}