ci(doc): fix github action by passing required RUSTDOCFLAGS

[xiangjinwu]

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What's changed and what's your intention?

See #23490 about the flag. The build.rustdocflags set in .cargo/config.toml is not effective when RUSTDOCFLAGS is already set.

Failures:
https://github.com/risingwavelabs/risingwave/actions/workflows/doc.yml

Fix verified:
https://github.com/risingwavelabs/risingwave/actions/runs/18774446293/job/53565733284

TODO:
Can we avoid the repetition?

Checklist

• I have written necessary rustdoc comments.
• I have added necessary unit tests and integration tests.
• I have added test labels as necessary.
• I have added fuzzing tests or opened an issue to track them.
• My PR contains breaking changes.
• My PR changes performance-critical code, so I will run (micro) benchmarks and present the results.
• I have checked the Release Timeline and Currently Supported Versions to determine which release branches I need to cherry-pick this PR into.

Documentation

• My PR needs documentation updates.

Release note