Cas Generate Overwrite Permission Denied

Fix permission denied when generated files overwrite CAS-materialized files

With the CAS enabled, Terragrunt fetches sources as read-only files. Writing a generated file over one of them no longer fails with permission denied:

• Files from generate blocks with if_exists = "overwrite", when the module ships the target file (for example, its own versions.tf).
• terragrunt.values.hcl, when the unit or stack source already contains one.
• terragrunt.autoinclude.hcl, when the unit or stack source already contains one.
• .terraform.lock.hcl, when the provider cache server updates a committed lock file during init -upgrade.

In each case, the read-only file is replaced with a writable one, and the shared CAS store is never modified.