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'))

Merge branch 'mnowak/linkcheck-disable-www-gnu-org' into 'main'

See merge request isc-projects/bind9!10436