deploy_nixos leaks files from the working directory into the world-readable nix store

[brainrake]

Describe the bug
deploy_nixos evaulates an expression that has ./. as src and leaks the contents of the working directory into the world-readable nix store. The working directory may contain (.gitignored) secrets so this is a security issue.
https://github.com/tweag/terraform-nixos/blob/646cacb12439ca477c05315a7bfd49e9832bc4e3/deploy_nixos/nixos-instantiate.sh#L22

To Reproduce
Use deploy_nixos module

Expected behavior
Don't leak files from working directory.

Environment

• OS name + version: NixOS unstable
• Version of the code: 646cacb