feat(ops/terraform): allow specifying an entrypoint for the attrset

This adds an additional parameter `entrypoint`, pointing to a .nix file
(or a directory containing a `default.nix` file) that's providing the
attribute path asked for.

If not set / kept at the default (empty string), it falls back to the
root dir of the repository as before.

Change-Id: I2e63114f21660c842153ac15424b3491d66624d2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8190
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>

ops/terraform/deploy-nixos/main.tf

@@ -24,6 +24,16 @@ variable "target_host" {
   type        = string
 }
 
+variable "entrypoint" {
+  description = <<EOT
+    Path to a .nix file (or directory containing `default.nix` file)
+    that provides the attrset specified in `closure`.
+    If unset, asks git for the root of the repository.
+  EOT
+  type        = string
+  default     = ""
+}
+
 variable "target_user" {
   description = "username on the target machine"
   type        = string
@@ -40,7 +50,8 @@ data "external" "nixos_system" {
   program = ["${path.module}/nixos-eval.sh"]
 
   query = {
-    attrpath = var.attrpath
+    attrpath   = var.attrpath
+    entrypoint = var.entrypoint
   }
 }