Bump minimist and mkdirp in /packages/system-ui-sass

Bumps [minimist](https://github.com/substack/minimist) and [mkdirp](https://github.com/isaacs/node-mkdirp). These dependencies needed to be updated together.

Updates `minimist` from 1.2.0 to 1.2.6
- [Release notes](https://github.com/substack/minimist/releases)
- [Commits](https://github.com/substack/minimist/commits)

Updates `mkdirp` from 0.5.1 to 0.5.6
- [Release notes](https://github.com/isaacs/node-mkdirp/releases)
- [Changelog](https://github.com/isaacs/node-mkdirp/blob/main/CHANGELOG.md)
- [Commits](isaacs/node-mkdirp@0.5.1...v0.5.6)

---
updated-dependencies:
- dependency-name: minimist
  dependency-type: indirect
- dependency-name: mkdirp
  dependency-type: indirect
...

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