fix(nimbus): change jetstream fetch to integer scheduling

[mikewilli]

Because

• jetstream fetch isn't firing
• jetstream fetch is the only scheduled task using crontab style schedule

This commit

• changes the jetstream fetch to integer-based schedule
• decreases the scheduled time from 6 to 4 hours to account for the fact that the integer-based schedule is interrupted by each deploy (i.e., first task fires 4 hours after deployment)

Fixes #11799