[9.20] chg: ci: Run system tests on all platforms on MR that touches system tests

Not doing this has lead to breakage caused by different dnspython
versions on different platforms only discovered in full nightly
pipelines.

Add a triggering rule for MRs changing code in bin/test/system.
Apply this rule to all nightly-only system test jobs.

Backport of MR !11214

Merge branch 'backport-stepan/run-all-system-tests-on-system-test-change-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11221