Cas Git Source Permission Denied

Fix permission denied when CAS fetches a git source across filesystems

With the CAS enabled, fetching a git:: source could fail with permission denied on .git/HEAD or .git/config, sending Terragrunt back to the standard getter. It happened when the CAS store and the module's working directory sit on different filesystems, so the files are copied rather than hard-linked, and a read-only leftover from an interrupted run was in the way. Terragrunt now recovers from the leftover and completes the fetch.