[github-actions] border_router test enhancements (#1206)
[github-actions] border_router test enhancements (#1206)

This commit enhances Border Router tests reliability:
- Set MAX_JOBS = 3
- Set fail-fast = false
- Cache test results for re-runs
openthread/ot-br-posix,openthread/borderrouter,openthread/ot-br-posix,openthread/ot-br-posix,openthread/ot-br-posix,openthread/borderrouter,openthread/ot-br-posix,openthread/borderrouter,openthread/borderrouter,openthread/borderrouter,openthread/borderrouter,openthread/ot-br-posix