Fix outputs' test and entrypoints, set PIP env vars

[mbargull]

Checklist

• Used a personal fork of the feedstock to propose changes
• Bumped the build number (if the version is unchanged)
• Reset the build number to 0 (if the version changed)
• Re-rendered with the latest conda-smithy (Use the phrase @conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• Ensured the license file is being packaged.

Addresses #14 (comment) and #14 (comment).

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[mbargull]

@carlodri

[mbargull]

Rerendering changes the README to say black-recipe/black-recipe-feedstock because it takes the toplevel name.
This does not affect the built packages, though. Those should still be only black+blackd.

[mbargull]

Build log shows that we now build without pip downloading things: https://dev.azure.com/conda-forge/feedstock-builds/_build/results?buildId=188051&view=logs&j=1bf226d3-0e2f-52d8-fa93-7d9e633347b3&t=c09a5a47-eec3-5700-9284-6f1bd7b23501&l=440

[carlodri]

very insightful PR, thank you @mbargull!