ci: use minimal stable toolchain for rust

[xushiyan]

Description

How are the changes test-covered

• [x] N/A
• [ ] Automated tests (unit and/or integration tests)
• [ ] Manual tests
  • [ ] Details are described below

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.26%. Comparing base (3e71bb0) to head (472c5f7).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #104 +/- ## ========================================== + Coverage 87.24% 88.26% +1.02% ========================================== Files 14 14 Lines 690 699 +9 ========================================== + Hits 602 617 +15 + Misses 88 82 -6 ```

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