install pymc in the recommended manner for remote workflows

[drbenvincent]

Following a discussion with @ricardoV94, it seems that we are pip installing pymc. We should instead install pymc in the recommended manner.

I'm not 100% sure how to implement this currently, but I'm assuming we need to edit files in .github/workflows. Perhaps...

https://github.com/pymc-labs/CausalPy/blob/83cb28cfd728ce01ed175c1ddd300b91308bf410/.github/workflows/ci.yml#L43

which may well require changes to the pyproject.toml file

https://github.com/pymc-labs/CausalPy/blob/83cb28cfd728ce01ed175c1ddd300b91308bf410/pyproject.toml#L29-L43