Use environment.yml (or something like that) in CI

[ricardoV94]

Description

The CI dependencies are completely dissociated from the conda pytensor-dev environment specified by environment.yml.

This led to need a separate commit in #448 that should have gone into #446

https://github.com/pymc-devs/pytensor/blob/a970407f5c3c4994d346a41065e335e89882dcfb/.github/workflows/test.yml#L139-L152

[lmmx]

The only tricky bit here is that you can't parameterise the Python version of an environment.yml, but we can do the installation of the parameterised Python version and the environments file in the same pass using this pattern