fix: Add clean target to debian/rules to ensure fresh builds

- Add override_dh_clean to remove build directory before each build
- Prevents stale CMakeCache.txt from causing ngl_codegen errors
- Ensures RELEASE_BUILD=ON is properly applied on each build

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

make_rpm_ubuntu26.py

@@ -257,6 +257,10 @@ def create_debian_rules(deb_dir, cmake_flags=""):
 %:
 	dh $@ --buildsystem=cmake --builddirectory=build
 
+override_dh_clean:
+	dh_clean
+	rm -rf build
+
 override_dh_auto_configure:
 	dh_auto_configure -- {cmake_opts}