Output the path to the Terraform binary

The path to the Terraform binary is important when using this action to configure Terraform for plugin testing as it needs to be set in the `TF_ACC_TERRAFORM_PATH` env variable to stop the binary being re-downloaded.

Ref: https://github.com/hashicorp/terraform-plugin-testing/issues/429

Start by reviewing how setup-terraform locates and exposes the Terraform binary, then trace how action outputs are defined and consumed. The change is complete when the action exposes the binary path so it can be assigned to TF_ACC_TERRAFORM_PATH and prevent Terraform from being downloaded again during plugin testing.

Written by the indexing model from the issue text.