Bump once_cell from 1.12.0 to 1.12.1 in /src/rust (#7392)

Bumps [once_cell](https://github.com/matklad/once_cell) from 1.12.0 to 1.12.1.
- [Release notes](https://github.com/matklad/once_cell/releases)
- [Changelog](https://github.com/matklad/once_cell/blob/master/CHANGELOG.md)
- [Commits](matklad/once_cell@v1.12.0...v1.12.1)

---
updated-dependencies:
- dependency-name: once_cell
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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Author: dependabot[bot]