merges manubot/rootstock#383

pandoc-eqnos upgrade addresses the following error:
UnboundLocalError: local variable 'ret' referenced before assignment
tomduck/pandoc-eqnos#43

Hold back pandoc due to the lack of panflute support.

Switch commands in build/README.md to be relative to the repo root.

merges manubot/rootstock#385

Pin panflute dependency
We use panflute to build custom pandoc filters. Panflute versions
only work with specific versions of the Pandoc AST.
We need to pin pandoc/panflute to a version pair that is known to
work together.

Pin versions of additional explicit manubot dependencies, as of
https://github.com/manubot/manubot/blob/01d69f066eaf6554111e6bcf197f7ee1fcfb240b/setup.py#L65-L80

CI: use conda-incubator/setup-miniconda on GH actions
https://github.com/conda-incubator/setup-miniconda
appears to have replaced https://github.com/goanpeca/setup-miniconda.
The later is currently 31 commits behind.

merges manubot/rootstock#389
closes manubot/rootstock#388

> The `set-env` command is deprecated and will be disabled on
November 16th. Please upgrade to using Environment Files.
For more information see:
https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

…otstock-2020-11-10