test(orchestrator): stabilize concurrent worker cap test

[setkyar]

The concurrent worker-cap test used /bin/sh with no arguments. That process exits immediately, allowing its reserved worker slot to be removed before all six requests reach the admission gate. Depending on scheduler timing (especially under -race), the test incorrectly counted more than three successful requests over time even though no more than three workers existed concurrently.

Use a temporary long-running worker script so all admitted slots remain occupied until assertions and cleanup.

Validation:

• target test under -race, 20 repetitions
• full ./scripts/run-go-race.sh
• git diff --check

[setkyar]

Superseded by consolidated dependency update #158, which includes this change and runs the complete CI suite once against the final combined dependency set.