Recognize nomadVar alloc index under job

### Proposal

Today, it's possible to store variable in Nomad and we can ACL's variable based on the path

```
{{ with nomadVar "nomad/jobs/group_name/task_name" }}
```

My proposal is to add a 4th layer of the alloc_index

```
{{ with nomadVar "nomad/jobs/job_name/group_name/task_name/1" }}
{{ with nomadVar "nomad/jobs/job_name/group_name/task_name/2" }}
{{ with nomadVar "nomad/jobs/job_name/group_name/task_name/${NOMAD_ALLOC_INDEX}" }}
```

### Use-cases

Having a group with multiple tasks don't means to have the same variables.

### Attempted Solutions

The only solution to keep ACL's working is to have the index in the variable names.

I can trick using something like

```
{{- define "custom" }}MY_VALUE_{{ NOMAD_ALLOC_INDEX }}{{ end }}
{{ executeTemplate "custom" }}: {{ env (executeTemplate "custom") }}
```

Because if you do

```
nomad/jobs/job_name/group_name/task_name/1
```

You lost the the ACL permission on the task name

Start by locating the nomadVar template entry point and reading how variable paths and ACL permissions are handled. Trace the existing job/group/task path behavior, then determine how alloc indices 1, 2, and NOMAD_ALLOC_INDEX should be recognized without losing task-level permissions. Done means the proposed paths work consistently and preserve the intended ACL scope.

Written by the indexing model from the issue text.