Set Terraform CLI options with the environment variable

#311

Some actions don't provide inputs to set Terraform CLI options, but you can set them with Terraform's environment variable TF_CLI_ARGS and TF_CLI_ARGS_name.

https://www.terraform.io/cli/config/environment-variables#tf_cli_args-and-tf_cli_args_name

e.g.

- uses: suzuki-shunsuke/tfaction/plan@latest
  with:
    github_token: ${{ secrets.GITHUB_TOKEN }}
  env:
    TF_CLI_ARGS_plan: "-parallelism=30"