MAINT: Lower pin Pixi 0.82.1 - #87

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v3, codecov/codecov-action@v4.0.1, prefix-dev/setup-pixi@v0.8.5. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/