Expand docs on code and variables in the flow.cylc. - #953

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: conda-incubator/setup-miniconda@3b0f2504dd76ef23b6d31f291f4913fb60ab5ff3. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Mamba support is still experimental and can result in differently solved environments!

WARNING conda.cli.condarc:set_key(521): Key auto_activate_base is an alias of auto_activate; setting value with latter

warning libmamba Security Warning: This transaction includes executing package scripts (pre/post-link/unlink) if present. These scripts can contain arbitrary code. Please ensure you trust the package sources.