e2etest: Consider removing ability to build from git ref

See https://github.com/hashicorp/terraform-exec/pull/305#discussion_r876039665.

This would simplify a part of the testing code that is not often used outside the nightly tests.

We'd still need to reproduce the binary caching functionality, or at least the ability to call different versions of the Terraform binary that have been pre-downloaded.

Start with the discussion on pull request #305 and trace the e2etest path that builds Terraform from a git ref. Review its nightly-test usage and the existing binary-caching behavior; done means simplifying or removing git-ref builds while preserving the ability to run pre-downloaded Terraform versions.

Written by the indexing model from the issue text.