Add back the element that yields app debug link

Adapt to changes in https://gitlab.com/freedesktop-sdk/freedesktop-sdk/-/commit/3867559fbb225abe36c47eb2f21ed73c58c30fac

Context: there is a symlink in runtime debug extension which allows app extension installed into /app to be accessible by gdb. Since runtime debug extension is gigabytes in size, runtime also contains a symlink in the directory into which runtime debug extension would be mounted to allow app debug extension to be used without it.