ci: install conda with setup-miniconda in the Linux conda jobs - #298

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@v4, BSFishy/pip-action@8f2d471d809dc20b6ada98c91910b6ae6243f318. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/