Bump pygments from 2.13.0 to 2.15.0 in /util/nix

Bumps [pygments](https://github.com/pygments/pygments) from 2.13.0 to 2.15.0.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](https://github.com/pygments/pygments/compare/2.13.0...2.15.0)

---
updated-dependencies:
- dependency-name: pygments
  dependency-type: direct:production
...

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