Flesh out CI, software environment, and tests.

[zaneselvans]

• Use correct conda environment in GitHub Actions workflow.
• Add python-snappy to the pudl-catalog conda environment.
• Add gcsfs to package dependencies.
• Add msgpack to package dependencies, even though in theory it should already be
  getting installed because it's a dependency of intake. Not sure what's going on there.
  Maybe @martindurant knows?
• Use the google storage location for the catalog by default if none is set by the user.
• Refer to the top level package constants for the known data locations elsewhere in the
  package.
• Re-write the tests to just use a small amount of remote data.
• Fix extras in tox.ini to refer to "tests" not "test"
• Pass PUDL_INTAKE_* environment variables in to Tox.