Consistent Terragrunt Cache

Consistent .terragrunt-cache directory generation

Terragrunt now creates a .terragrunt-cache directory for every run, regardless of whether the terragrunt.hcl file defines a terraform block with a source attribute.

This change improves consistency across all Terragrunt executions, as OpenTofu/Terraform will now always run within the .terragrunt-cache directory. This standardized behavior simplifies troubleshooting and makes the execution model more predictable.