"worker agent experienced a fatal error; aborting job" during Omicron CI run

[hawkw]

During a CI run for oxidecomputer/omicron#7437, the buildomat / build and test (helios) job failed mid-run with the message "control: worker agent experienced a fatal error; aborting job". Complete logs from that test run can be found here, but they're not terribly interesting; it's just compiling Nexus happily when it abruptly logs "worker agent experienced a fatal error; aborting job" and the job ends.

I'd be happy to help track down any additional information that would be helpful for debugging this, but I'm not sure where I'd look. Let me know if anything else would be useful?