Strange behavior when used with some nomad interpolation calling filesystem function

Hello,

I'm not fluent at all in go and go template and the worst of all I'm quite new to nomad but I found the following behavior strange.

I have a .nomad.tpl file for nomad pack with the following:

```
job "hello_pack" {
…
group "app" {
…
task "server" {
driver = "docker"
…
config {
…
volumes = ["${abspath(".")}/packs/hello_pack:/app"]
}
}
}
}

```

The aim is to mount a relative folder. (I took my inspiration from there https://github.com/hashicorp/nomad/issues/9369#issuecomment-728929474)

Result:
- nomad-pack complains when I try to do a `nomad-pack run`
```
! Failed To Parse Job Specification

Error: input.hcl:122,23-31: Error in function call; Call to function "abspath" failed: filesystem function disabled., and 1 other diagnostic(s)

```

- but when I **render** the final hcl file with `nomad-pack render`, the result is **valid for nomad** and so, the rendered job can be run with the `nomad job run` command.

The behavior seems odd to me (why in one case it's allowed and disallowed in the other?). I would expect the same result (both running).

Version numbers:
- Nomad v1.2.6 (a6c6b475db5073e33885377b4a5c733e1161020c)
- Nomad Pack v0.0.1-techpreview3 (568ac5e)

Reproduce the issue with the provided .nomad.tpl example, comparing `nomad-pack run` with `nomad-pack render` followed by `nomad job run`. Start by tracing the two command entry points and how each parses the `abspath` filesystem function; done means the behavior is consistent or its intended difference is clearly documented.

Written by the indexing model from the issue text.