feat(ops/terraform): add trigger to deploy-nixos, remove target_name

This allows passing in custom triggers to trigger a (re)deploy.

For example, a caller can put an AWS instance ID into the triggers to
cause a redeploy whenever the instance ID has changed.

The `target_name` terraform variable was doing something similar, but
`triggers` is more generic, allowing multiple triggers, without having
to stringify them.

We also don't need to trigger on the attrpath - it can be changed, and
as long as it still evaluates to the same
`data.external.nixos_system.result.drv` (which is checked on every
plan), no redeploy needs to be made.

Change-Id: I94ce787a50830b87b6f53c08e042e4abe4036bdd
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8191
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: flokli <flokli@flokli.de>

ops/terraform/deploy-nixos/README.md

@@ -21,7 +21,6 @@ deploy is necessary.
 module "deploy_somehost" {
   source              = "git::https://code.tvl.fyi/depot.git:/ops/terraform/deploy-nixos.git"
   attrpath            = "ops.nixos.somehost"
-  target_name         = "somehost"
   target_host         = "somehost.tvl.su"
   target_user         = "someone"
   target_user_ssh_key = tls_private_key.somehost.private_key_pem