feat: Added env var support

[vfoucault]

This pr is meant to add the support of passing Environment variables to a specific terraform-exec run.

When you want to exec multiple runs in parallel that you can't manage, and using os.Environ directly is not an option

• to be used like

var options = []tfexec.PlanOption{
  tfexec.EnvVar("SOME_ENV_VAR", "SOME_VALUE"),
  tfexec.EnvVar("OTHER_ENV_VAR", "OTHER_VALUE"),
}

[hashicorp-cla]

All committers have signed the CLA.