fix(ci): clear stable toolchain override for Dylint

[zackees]

The first merged-commit Dylint run exposed an ambient selector from setup-soldr: it exports RUSTUP_TOOLCHAIN=1.95.0, which takes precedence over the lint libraries' nightly-2026-05-28 manifests inside soldr dylint prepare. Soldr therefore derived unpublished nightly-2026-02-28 and attempted a driver source build.

Clear RUSTUP_TOOLCHAIN only for Dylint preparation and execution. Keep the stable pin for fbuild's normal jobs and explicit nightly pins for lint formatting and UI tests. The local lint entry point applies the same rule.

Validation: added a RED -> GREEN regression test for inherited stable RUSTUP_TOOLCHAIN; 21 workflow/pin/local lint tests pass, Ruff passes, and git diff --check passes.

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between f551aaa and b24f83d.

📒 Files selected for processing (3)

• .github/workflows/dylint.yml
• ci/run_dylint.py
• ci/test_run_dylint.py

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