Use `pipenv` to install the `cisagov/guacscanner` package

[mcdonnnj]

💡 Summary

So that we can control all dependencies it would be helpful to install the cisagov/guacscanner package using Pipenv.

Motivation and context

This would help ensure repeatable builds as defined in the Docker configuration.

Implementation notes

A similar approach was taken in cisagov/vdp-scanner-docker.

Acceptance criteria

• Pipenv is used to install the cisagov/guacscanner package and its dependencies.