fix: pin checkout v6.0.3 for fork PR workflows

[aptend]

Summary

Temporarily pin the actions/checkout steps that fetch a pull request's head repository to the last known working v6.0.3 commit:

df4cb1c069e1874edd31b4311f1884172cec0e10

Only checkout steps using github.event.pull_request.head.repo.full_name are pinned. Checkouts of trusted fixed repositories such as matrixorigin/mo-tester and matrixorigin/CI remain on @v6.

Background

actions/checkout@v6 was a floating major-version reference. On 2026-07-20, GitHub published v6.1.0 and moved the v6 tag from df4cb1c to d23441a.

v6.1.0 introduced a breaking security default that rejects checking out fork PR code from a pull_request_target workflow unless allow-unsafe-pr-checkout is explicitly enabled. MatrixOne's PR entrypoint uses pull_request_target, inherits secrets, and delegates to these reusable workflows, which explicitly check out the fork repository and branch.

As a result, all fork PR jobs now fail during checkout before any MatrixOne code is built or tested. Example: matrixorigin/matrixone#25909, Actions run https://github.com/matrixorigin/matrixone/actions/runs/29756799570.

A successful job immediately before the release resolved actions/checkout@v6 to df4cb1c; a failing job 34 minutes later resolved the same reference to d23441a, with the same runner image and CI workflow revision.

Scope

Pin 12 PR-head checkout steps across:

• .github/workflows/ci.yaml
• .github/workflows/e2e-upgrade.yaml
• .github/workflows/e2e-compose.yaml
• .github/workflows/e2e-standalone.yaml
• .github/workflows/utils.yaml

This covers UT, SCA, upgrade, Compose/Standalone BVT, coverage, and benchmark workflows for fork PRs targeting MatrixOne main.

Security note and follow-up

This is an explicit temporary rollback to the pre-v6.1.0 behavior. It restores fork PR CI but also preserves the existing risk of executing fork-controlled code in a pull_request_target context with inherited secrets and privileged runners.

The durable fix is to split the model:

• run untrusted PR code under pull_request with read-only permissions, no inherited secrets, and isolated runners;
• keep pull_request_target for metadata/label/authorization work only, without checking out PR code;
• gate genuinely privileged tests by exact head SHA and run them with short-lived, least-privilege credentials in an isolated environment.

Do not replace this pin with allow-unsafe-pr-checkout: true as the long-term solution.

Validation

• Confirmed the pinned SHA is the v6.0.3 release commit.
• Confirmed every checkout step that uses github.event.pull_request.head.repo.full_name is pinned.
• Confirmed fixed-repository checkout steps remain unchanged on @v6.
• git diff --check
• actionlint passed for the changed workflows after ignoring existing custom-runner-label and pre-existing expression-schema findings.