Update nightly toolchain for cargo-udeps CI job

[JoshuaBatty]

Description

Point cargo-unused-deps-check at nightly-2025-08-15, which ships rustc 1.88 so cargo-udeps and its cargo dependencies install cleanly. This should fix the currently failing job.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.