Simplify .hgscfg management

[petebachant]

This change tracks the creation of the .hgscfg file in the pipeline, taking it out of the repo, and running it once per machine if it doesn't exist. It also puts a copy in the modules directory so the notebook can use it when running interactively.

The env var deps are also updated to a newer format.

[rebeccamccabe]

this looks good, although the action is not running, do I need to give you a permission somewhere?

[petebachant]

this looks good, although the action is not running, do I need to give you a permission somewhere?

I believe that by default, Actions from forks do not run for security's sake (docs).