ci: Use ignore_scheduled_pipelines anchor in .radeonsi-rules

ci: Use ignore_scheduled_pipelines anchor in .radeonsi-rules

Without this, scheduled pipelines erroneously tried to create jobs using
this template, which can't work, because their dependency jobs don't
exist in scheduled pipelines. Unfortunately, this resulted in scheduled
pipelines not running silently, without any direct feedback about what's
wrong (see https://gitlab.com/gitlab-org/gitlab/-/issues/25490).

Fixes: 6c8b921572a5 "ci: Build kernels and rootfs for x86 devices"
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6802>