Handle modification of gwdatafind_server/_version.py by "debian/rules clean"

On some build platforms (this time, Ubuntu Focal and Jammy), debian/rules clean may modify gwdatafind_server/_version.py in a way that makes the unpacked tarball look no longer pristine. To fix this, the relative path of the affected file needs to be added to a (new) file debian/clean - I seem to be unable to use the WebIDE to create a MR (browser issue??).

1.3.0-1.1 Debian/Ubuntu builds successfully use the suggested change.