Legacy Internal Tflint

Force the use of external tflint binary.

legacy-internal-tflint - Reason

The embedded version of tflint has been held back by upstream's adoption of the BUSL license. As a result, the embedded version is horribly out of date, and we are deprecating it for removal in a future version of Terragrunt. You may use --terragrunt-external-tflint in your tflint hook to opt in to the use of an external tflint binary, or enable this strict control.