Temporarily disable soak testing (#10084)

In #10053 we're attempting to diagnose why soak tests are not reliably producing
artifacts in CI. This work is made harder by competition from other jobs. With
the utility of soaks being low it makes sense to disable these for now, to be
re-enabled once the experiment reporting is reliable again.

Signed-off-by: Brian L. Troutwine <brian@troutwine.us>