fix(prow): normalize goproxy for prow-jobs/pingcap/tidb-engine-ext

[Copilot]

Remove internal goproxy.pingcap.net endpoint from tidb-engine-ext presubmit jobs, replacing it with the official public Go module proxy.

Changes

• latest-presubmits.yaml / release-6.1-latest-presubmits.yaml
  • Replace internal proxy http://goproxy.pingcap.net,direct → https://proxy.golang.org,direct
  • Fix env var name: GO_PROXY → GOPROXY (standard Go env var)

# Before
- name: GO_PROXY
  value: http://goproxy.pingcap.net,direct

# After
- name: GOPROXY
  value: https://proxy.golang.org,direct

Original prompt

---

This section details on the original issue you should resolve

<issue_title>goproxy: normalize proxies for prow-jobs/pingcap/tidb-engine-ext</issue_title>
<issue_description>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
  </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes goproxy: normalize proxies for prow-jobs/pingcap/tidb-engine-ext #4239

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[ti-chi-bot]

Hi @Copilot. Thanks for your PR.

I'm waiting for a PingCAP-QE member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[ti-chi-bot]

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