[terraform] Add workspace check

Summary:
Resolves ENG-4629.
After nixpkgs upgrade, we have Terraform 1.5 which allows to add check blocks.
This allows us to display a user-friendly error message if the workspace is not set.

Test Plan:

terraform workspace select default
terraform plan

Reviewers: michal, kamil, varun, jon

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9046