[9.18] chg: ci: Disable linkcheck on www.gnu.org

The check fails with the following error for some time:

    broken    https://www.gnu.org/software/libidn/#libidn2 - HTTPSConnectionPool(host='www.gnu.org', port=443): Max retries exceeded with url: /software/libidn/ (Caused by NewConnectionError('<urllib3.connection.HTTPSConnection object at 0x7f5bd4c14590>: Failed to establish a new connection: [Errno 111] Connection refused'))

Backport of MR !10436

Merge branch 'backport-mnowak/linkcheck-disable-www-gnu-org-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!10438