Terragrunt non-interactive arguments/env var changes.

[kaorihinata]

Describe the bug
Arguments/environment variables for Terragrunt support are changing which is causing warnings to appear at the top of Terragrunt output:

07:23:17.251 WARN   The `--terragrunt-non-interactive` flag is deprecated and will be removed in a future version of Terragrunt. Use `--non-interactive` instead.
07:23:17.251 WARN   The `TERRAGRUNT_FORWARD_TF_STDOUT` environment variable is deprecated and will be removed in a future version of Terragrunt. Use `TG_TF_FORWARD_STDOUT=true` instead.

This may also be a good opportunity to replace a flag with an environment variable (or maybe even remove the Terragrunt check at line 195 of internal/task/task.go entirely if you don't mind the extra environment variables.)

• --terragrunt-non-interactive has been replaced with --non-interactive, but we also have TERRAGRUNT_NON_INTERACTIVE, or the new flag which does not produce a warning, TG_NON_INTERACTIVE.
• TERRAGRUNT_FORWARD_TF_STDOUT is also being replaced with TG_TF_FORWARD_STDOUT.

For the environment variables, providing both at the same time doesn't seem to be a problem, and the older TERRAGRUNT_* forms go quite far back (TERRAGRUNT_FORWARD_TF_STDOUT goes back to the output changes that made it necessary in the first place, and TERRAGRUNT_NON_INTERACTIVE has been there for years.)

I tested the following on internal/task/task.go. It removed the warnings and should cover older versions as well:

@@ -195,4 +195,8 @@
 	if task.Program == "terragrunt" && f.terragrunt {
+		// Flags for older Terragrunt.
 		task.AdditionalEnv = append(task.AdditionalEnv, "TERRAGRUNT_FORWARD_TF_STDOUT=1")
-		task.Args = append(task.Args, "--terragrunt-non-interactive")
+		task.AdditionalEnv = append(task.AdditionalEnv, "TERRAGRUNT_NON_INTERACTIVE=1")
+		// Flags for newer Terragrunt.
+		task.AdditionalEnv = append(task.AdditionalEnv, "TG_TF_FORWARD_STDOUT=1")
+		task.AdditionalEnv = append(task.AdditionalEnv, "TG_NON_INTERACTIVE=1")
 	}

Setup (please complete the following information):

• OS [e.g. Ubuntu, macOS] macOS Sonoma (15.5)
• Shell [e.g. zsh, fish] zsh
• Terminal Emulator [e.g. kitty, iterm] iTerm
• Terminal Multiplexer [e.g. tmux] tmux

To Reproduce

• Create a simple terraform module with an empty terragrunt.hcl file.
• Using pug, plan a run of that module.

Expected behavior
Terragrunt output without deprecation warnings.

[kaorihinata]

Looking around, this will also require a small adjustment to the README as it does mention how pug interacts with Terragrunt. I wasn't sure whether to call this a bug or an enhancement as it's avoiding future issues, so if you'd rather I file things like this as an enhancement in the future then let me know.