build-debian-copyright: output relative pathnames
build-debian-copyright: output relative pathnames

236a9bc8ff91ba300ae11b523c4229469b893c1d inadvertently changed the
output format by switching over to using the full pathname of the files
to scan.

Rather than trying to back that out, let's prefer to directly do what we
want here: force the output to be a relative path name starting at the
toplevel directory.

Fixes #17164

mvollmer/cockpit,mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit,martinpitt/cockpit,mvollmer/cockpit,mvollmer/cockpit,mvollmer/cockpit,martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,martinpitt/cockpit