We can't add -var-file to an apply command if a plan is specified.

OpenTofu 1.8 allows using the `-var-file` switch in almost all of the
commands. Terraform does not. Add support if not using Terraform.

Add the `TF_CLI` environment variable to use a different terraform
binary (like opentofu) and some documentation.

The script is copied from the rcfiles repo and the rest is copied from
the sampleproject repo.