set up stable Rust toolchain on all rust builds

[davidhewitt]

Change Summary

Should fix the build failures observed in CI related to the recently-stabilized OnceLock.

Related issue number

N/A

Checklist

• Unit tests for the changes exist
• Documentation reflects the changes where applicable
• Pydantic tests pass with this pydantic-core (except for expected changes)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

Selected Reviewer: @adriangb

[davidhewitt]

please review

[samuelcolvin]

LGTM

[codecov]

Codecov Report

Merging #782 (ce66bb1) into main (9be711e) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #782   +/-   ##
=======================================
  Coverage   93.67%   93.67%           
=======================================
  Files          99       99           
  Lines       14290    14290           
  Branches       25       25           
=======================================
  Hits        13386    13386           
  Misses        898      898           
  Partials        6        6           

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[codspeed-hq]

CodSpeed Performance Report

Merging #782 will not alter performance

_{Comparing dh/fix-ci (ce66bb1) with main (9be711e)}

Summary

✅ 126 untouched benchmarks