fix: Interpolate environment variables

[KyleTryon]

Changes

• fix: Interpolates environment variables within backend_config and var
• ci: added the GHI token to enable PR comments

Fixes

• Circle CI context environment variables not being interpolated in 3.0.0 #51
• Terraform init with S3 backend is failing with terraform version 0.15+ and orb 3.0.0 #50
• Path test for tfvar files should accept paths outside of the module_root #56

[KyleTryon]

A preview is ready if anyone would be willing to test it out in the wild before publication. This change does include the other minor version changes currently staged, though all should be backwards compatible as well.

---

Your development orb has been published. It will expire in 30 days.
You can preview what this will look like on the CircleCI Orb Registry at the following link:
https://circleci.com/developer/orbs/orb/circleci/terraform?version=dev:997fc3ea0f586a39be65e7c800118ba77669c262

[orb-publisher]

Your development orb has been published. It will expire in 30 days.
You can preview what this will look like on the CircleCI Orb Registry at the following link:
https://circleci.com/developer/orbs/orb/circleci/terraform?version=dev:f24a25f326c5dd031d1ed44800e059a5b597ffc2

[orb-publisher]

Your development orb has been published. It will expire in 30 days.
You can preview what this will look like on the CircleCI Orb Registry at the following link:
https://circleci.com/developer/orbs/orb/circleci/terraform?version=dev:f588330bf6b332f4679e6ffb31a81eba2679acfe