Set a environment variable to indicate when running quartodoc build

[machow]

from @gadenbuie, pkgdown sets the environment variable IN_PKGDOWN="true" when running. We could similarly set an environment variable.

This is useful for packages that dynamically modify docstrings (or other behavior), but only when generating API docs. For example, plotnine has a bunch of example notebooks it attaches, and py-shiny has shinylive examples it attaches to API docs.