feat(ops/terraform/deploy-nixos): make target_user_ssh_key optional

In case `target_user_ssh_key` points to an empty string, nixos-copy.sh
just doesn't set `IdentityFile=` at all.

This allows using deploy-nixos without any explicitly passed ssh keys,
but picking up whatever ssh setup the user has configured locally.

Change-Id: If335ce8434627e61da13bf6923b9767085af08a5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8576
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

ops/terraform/deploy-nixos/main.tf

@@ -37,6 +37,7 @@ variable "target_user" {
 variable "target_user_ssh_key" {
   description = "SSH key to use for connecting to the target"
   type        = string
+  default     = ""
   sensitive   = true
 }