terraform_cli: support path and env and in terraform_cli blocks

[ryancragun]

At some point during the serial to multiplex rewrite of Enos, we stopped
respecting the special path and env attributes in terraform_cli
blocks.

I discovered this when attempting to test an older version of Terraform
with enos by setting the path and it was still resolving the latest
version which it found via the PATH.

This resolves that by configuring the Terraform executor in the
workspace with any special terraform_cli variables that have been set
in the flight plan.

• Configure the Terraform executor with the scenario "terraform_cli"
  attributes if necessary
• Regenerate protobuf with latest generator
• Update Go modules
• Bump version

Checklist

• The commit message includes an explanation of the changes
• Manual validation of the changes have been performed (if possible)
• New or modified code has requisite test coverage (if possible)
• I have performed a self-review of the changes
• I have made necessary changes and/or pull requests for documentation
• I have written useful comments in the code