Implement corefunc_env_copy_to_var

[skyzyx]

Go: corefunc.EnvCopyToVar(envName, tfVarName)
TF: corefunc_env_copy_to_var

Real-world use-case: When integrating PagerDuty and Datadog, need to read PagerDuty API key from PAGERDUTY_TOKEN, then pass it to a variable, so that the Datadog provider can read it without printing in text (which is insecure).

If a Terraform variable is called var.pagerduty_token, then this would read PAGERDUTY_TOKEN and then write that same value to TF_VARS_pagerduty_token so that Terraform will read it as the variable value.

variable "pagerduty_token" {
  description = "PagerDuty API token. Value is set via .env file."
  type        = string
}

data "corefunc_env_copy_to_var" "pagerduty_token" {
  name     = "PAGERDUTY_TOKEN"
  variable = "pagerduty_token"
}

Yes, you could do export TF_VARS_pagerduty_token=$PAGERDUTY_TOKEN, but:

1. You'd be surprised how few people know about this.
2. This (and corefunc_env_ensure) help make it an explicit and unmissable part of running the Terraform.

[skyzyx]

Meh. The copy needs to happen before Terraform is run. Boo.