fix(ci): set CARGO_BUILD_JOBS=2 for unit tests

[hanabi1224]

Summary of changes

This PR tries to mitigate cargo test build timeout issue.

Changes introduced in this pull request:

Reference issue to close (if applicable)

Closes

Other information and links

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation. All new code adheres to the team's documentation standards,
• I have added tests that prove my fix is effective or that my feature works (if possible),
• I have made sure the CHANGELOG is up-to-date. All user-facing changes should be reflected in this document.

Outside contributions

• I have read and agree to the CONTRIBUTING document.
• I have read and agree to the AI Policy document. I understand that failure to comply with the guidelines will lead to rejection of the pull request.

Summary by CodeRabbit

• Chores
  • Centralized build flags in the CI environment to reduce overall compile times.
  • Reduced parallel build concurrency to improve stability on hosted runners.
  • Added inline comments in the workflow for clarity and future maintenance.
  • Preserved existing test-related environment variables and settings.

[coderabbitai]

Walkthrough

Moves RUSTFLAGS to the workflow top-level env, removes per-step RUSTFLAGS, changes CARGO_BUILD_JOBS from 3 to 2 and moves it into the unit-tests run step with a hosted-runners comment, and preserves other existing env vars in .github/workflows/unit-tests.yml.

Changes

Cohort / File(s)	Summary
GitHub Actions workflow .github/workflows/unit-tests.yml	Added global RUSTFLAGS in workflow env; removed per-step RUSTFLAGS; changed CARGO_BUILD_JOBS from 3 to 2 and moved it into the unit-tests run step with a hosted-runners comment; kept other env vars unchanged.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• fix(test): speed up debug and codecov tests #6340: Also moves/adjusts RUSTFLAGS placement in CI workflow files.
• fix(ci): reduce logs in CI tests to speed up the jobs #6382: Edits the unit-tests workflow environment and touches RUSTFLAGS handling.
• fix(ci): disable debug tests #6357: Modifies the unit-tests workflow with RUSTFLAGS and related env changes.

Suggested reviewers

• akaladarshi
• sudo-shashank
• LesnyRumcajs

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Title check	✅ Passed	The title references setting CARGO_BUILD_JOBS=2, which matches the main change in the changeset (updating CARGO_BUILD_JOBS from 3 to 2 in the CI workflow).

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch hm/CARGO_BUILD_JOBS-1-for-ut

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