Add MyST compatibility

[brian-rose]

This PR updates the build-book.yaml workflow in two important ways:

• uses micromamba to set up the execution environment (much faster) and gets rid of the environment caching (no longer needed with micromamba performance)
• Add a new input argument base_url which is necessary for building with MyST

The calling workflow needs to specify build_command: 'myst build --execute --html' (and include mystmd in its environment file) for this to work with MyST.

Everything here should be backwards compatible but we will have to do some manual checking that existing non-MyST builds still work (#6 still unresolved).

[brian-rose]

I will merge and do some manual testing.