goproxy: normalize proxies for prow-jobs/pingcap/tidb-engine-ext

[ti-chi-bot]

Replace internal goproxy endpoints for prow-jobs/pingcap/tidb-engine-ext with official/public proxies.

Acceptance criteria:

• No internal goproxy endpoints in prow-jobs/pingcap/tidb-engine-ext
• GOPROXY values set to default or official proxies