Weird error with Nomad Variables and templates when trying to toJSONPretty

### Nomad version
`Nomad v1.5.3` (client & server)

### Issue
I'm trying to generate a file via `template{}` that has the exact same contents as my Nomad Variable.

Here's my variable data at `nomad/jobs/somejob`:
```json
{
"baz": "qux",
"foo": "bar"
}
```

Here's my template
```hcl
template {
data = <<-EOF
{{ nomadVar "nomad/jobs/somejob" | toJSON }}
EOF
destination = "local/output"
}
```
The above works just as expected, and the contents of `local/output` is:

```json
{"baz":"qux","foo":"bar"}
```

However, when switching `toJSON` to `toJSONPretty` I get the following error (allocation):

```
Template failed: (dynamic): execute: template: :2:7: executing "" at : wrong type for value; expected map[string]interface {}; got *dependency.NomadVarItems
```

Which is super weird because why would a pretty json be any different from just regular json?

I went ahead and tested it with latest consul-template (v0.31.0) and it seems to work just fine:

```shell
$ cat in.tpl
{{ with nomadVar "nomad/jobs/somejob" }}
{{ . | toJSONPretty }}
{{ end }}

$ consul-template -once -dry -template=in.tpl
> out.json

{
"baz": "qux",
"foo": "bar"
}
```

### Reproduction steps

1. Add variables to `nomad/jobs/somejob`
2. Run the job bellow (uncomment to see it breaking):

```hcl
job "somejob" {
group "somejob" {
task "somejob" {
driver = "docker"

config {
image = "busybox:latest"
command = "sh"
args = ["-c", "while true; do echo . && sleep 5; done"]
}

template {
data = <<-EOF
{{ nomadVar "nomad/jobs/somejob" | toJSON }}
EOF

destination = "local/flat.json"
}

# Uncomment below to let it start failing
#
# template {
# data = <<-EOF
# {{ nomadVar "nomad/jobs/somejob" | toJSONPretty }}
# EOF

# destination = "local/pretty.json"
# }
}
}
}
```

#### Expected Result
`toJSON` and `toJSONPretty` working the same when combined with `NomadVarItems`.

#### Actual Result
`toJSONPretty` doesn't work (only in Nomad).

#### Workarounds

```
{{ nomadVar "path" | toJSON | parseJSON | toJSONPretty }}
```

Start with the provided Nomad job reproduction and compare the template behavior of toJSON and toJSONPretty when given nomadVar output. Confirm the issue against Nomad v1.5.3 or a current build, then trace the template function handling; done means both functions accept NomadVarItems consistently without requiring the parseJSON workaround.

Written by the indexing model from the issue text.