fix: authenticate Terraform with GOOGLE_APPLICATION_CREDENTIALS

Write GCP_SERVICE_ACCOUNT_KEY to /tmp/gcp-key.json and export
GOOGLE_APPLICATION_CREDENTIALS so the Google provider authenticates
automatically without requiring gcloud auth application-default login.

Also export TF_VAR_project_id from GCP_PROJECT_ID so the variable
is passed to Terraform without needing -var flags in the script.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>