Plan too long to be passed between steps

[This example in the documentation](https://github.com/hashicorp/setup-terraform/blame/master/README.md#L112) often breaks down in real life. If the output from `terraform plan` is too big, it can't fit in an environment variable (which is how [the documentation shows it being accessed](https://github.com/hashicorp/setup-terraform/blame/master/README.md#L123)).

The workaround we have found is to save the plan as a file, pass the filename, and then use `terraform show -json $filename > plan.json` to get the plan value in the second step. The problem we are having with this workaround is that the wrapper script breaks STDOUT.