bake: allow calling env as input var default value

[crazy-max]

Following #192 and specially this code portion, I don't think that's the proper way to handle environment variable as default value because this is obfuscated and not obvious:

variable "TAG" {
  default = ""
}

Instead we should create a function called env to handle that:

variable "tag" {
  default = env("TAG")
}

[crazy-max]

Closed in favor of #541