dev/ci: assign to job agents on rebuilds instead of retries

Created by: bobheadxi

Another pass at https://github.com/sourcegraph/sourcegraph/pull/32755 - retried builds don't count as incrementing retry counts, and there's no way to change the pipeline step on the fly to target a new queue for specific job retries, so we cant change the behaviour based on retry count. However, I confirmed this variable is set on a rebuild, so we can use this instead to re-run entire builds on job-based agents

Test plan

n/a