feat(nimbus): results_ready should be based off of the computed enrollment

[mikewilli]

Because

• enrollment can end before proposed enrollment duration is over
• results link is not available until results_ready_date + 8 days
• results_ready_date is (was) based off of the proposed enrollment
• proposed enrollment does not change to reflect actual enrollment

This commit

• updates the results_ready_date to use the actual (if available) or computed enrollment

Fixes #10559