[github-actions] pin to Thread Version 1.2 (#1376)
[github-actions] pin to Thread Version 1.2 (#1376)

Until the test scripts are updated to support Thread Version 1.3.
openthread/ot-br-posix,openthread/borderrouter,openthread/borderrouter,openthread/ot-br-posix,openthread/ot-br-posix,openthread/borderrouter,openthread/ot-br-posix,openthread/borderrouter,openthread/borderrouter,openthread/ot-br-posix,openthread/ot-br-posix,openthread/borderrouter