Add required `TF_VAR_domain` to sample.sh
Add required `TF_VAR_domain` to sample.sh

Since the `domain` variable is required for the test fixture module, we
should indicate that it needs to be set in the sample environment
variables.

terraform-google-modules/terraform-google-project-factory,terraform-google-modules/terraform-google-project-factory,terraform-google-modules/terraform-google-project-factory,terraform-google-modules/terraform-google-project-factory