[9.18] chg: ci: Run linkchecker only on Wednesdays

Some domains tested by linkchecker may think that we connect to them too
often and will refuse connection or reply with an error code, which makes
this job fail. Let's check links only on Wednesdays.

Backport of MR !10439

Merge branch 'backport-mnowak/run-linkchecker-only-sometimes-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!10441