Set TF_WORKSPACE in terraform-pipeline.yml

It was pointed out by @rjg21 that there can sometimes be issues with initialising terraform in the default workspace when the default workspace is not actually in use (as we often have configured). This is also mentioned in the docs https://developer.hashicorp.com/terraform/tutorials/automation/automate-terraform#multi-environment-deployment.

Therefore, this pipeline needs to be updated so that the TF_WORKSPACE variable is set on each job that runs a terraform command to ensure the correct workspace is selected for all commands.