Set TERRAFORM_BINARY_NAME for cdktf

[matschaffer-roblox]

Not sure if this is possible, but it looks like cdktf writes outside of the current project directory which makes using a local .tool-versions a little tricky.

hashicorp/terraform-cdk#1559 has some info

If this plugin were able to export (TERRAFORM_BINARY_NAME=(local version) before invoking the shim that might provide a transparent workaround.

Feel free to close if this isn't the sort of thing asdf is able to do. I've never tried to create a plugin for it.