[9.18] fix: ci: Don't forward parent yaml variables to stress child pipelines

The global RUNNER_SCRIPT_TIMEOUT: 55m in the parent pipeline was being
forwarded to the stress and tsan:stress child pipelines, where forwarded
yaml variables outrank job-level variables. That caused stress jobs with
BIND_STRESS_TESTS_RUN_TIME >= 60 to be killed at 55 minutes, regardless
of the per-job RUNNER_SCRIPT_TIMEOUT set in the generated child config.

Set forward:yaml_variables: false on both trigger jobs; the generated
configs already declare every variable they need.

Assisted-by: Claude:claude-opus-4-7
Backport of MR !12012

Merge branch 'backport-mnowak/fix-stress-test-script-timeout-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!12016