Update udeps installation.

[ilyalesokhin-starkware]

Type

• feature
• bugfix
• dev (no functional changes, no API changes)
• fmt (formatting, renaming)
• build
• docs
• testing

Description

Breaking changes?

• yes
• no

---

This change is 

[cursor]

PR Summary

Low Risk
CI-only workflow change that affects toolchain selection for installing cargo-udeps, with no runtime or library code impact.

Overview
The udeps GitHub Actions job now explicitly installs the configured stable toolchain and uses cargo +${{ env.STABLE_VERSION }} to install cargo-udeps, avoiding reliance on whatever default toolchain is present.

The actual cargo +${{ env.NIGHTLY_VERSION }} udeps run remains on nightly; only the tool installation path changes.

^{Reviewed by Cursor Bugbot for commit 5857541. Bugbot is set up for automated code reviews on this repo. Configure here.}

[ilyalesokhin-starkware]

doesn't work