Toggle variable substitution per `values` element.

### Is your feature request related to a problem? Please describe
When setting `values` within a uds-bundle.yaml, it is sometimes required that a literal environment variable key is passed on as an override. For example, this happens to be the case for Loki. In Loki, when the `config.expand-env` argument is called for the Loki binary, Loki will then expand references to environment variables (values matching the same [regex](https://github.com/defenseunicorns/uds-cli/blob/1761f29c9031c913435641fee7fb92bb77ea7880/src/pkg/bundle/overrides.go#L29) syntax as we use in UDS CLI) and use their values at runtime. However, if today you attempt to set the value of a helm values path to something like `"${EXAMPLE}"`, you would see the override value rendered as `${EXAMPLE_not_found}` because you did not set `${EXAMPLE}` with `UDS_EXAMPLE`, or with `--set`, or in `uds-config.yaml`.

Because it's a bit confusing to describe in words, here is what I want in the final configmap, after UDS has deployed my bundle/package/helm chart:
```yaml
kind: ConfigMap
metadata:
name: example-data
data:
myconf.yaml: |
some-setting: "${EXAMPLE}"
```

And here's how I'd like to supply that value to the configmap via uds-bundle:
```yaml
values:
- path: path.to.myconf.some-setting
value: "${EXAMPLE}"
```

Today I get

```
kind: ConfigMap
metadata:
name: example-data
data:
myconf.yaml: |
some-setting: "${EXAMPLE_not_found}"
```

### Describe the solution you'd like

- **Given** a state
- **When** an action is taken
- **Then** something happens

Given the presence of a value in uds-bundle.yaml that matches the [regex](https://github.com/defenseunicorns/uds-cli/blob/1761f29c9031c913435641fee7fb92bb77ea7880/src/pkg/bundle/overrides.go#L29), when I set the attribute `substitute` to `false` for the value matching the regex, with would like the UDS CLI to NOT attempt to substitute the value of the `value` key. E.g.

```yaml
values:
- path: path.to.myconf.some-setting
value: "${EXAMPLE}"
substitute: false
```

### Describe alternatives you've considered
You can currently work around this by turning the `values` element into a `variables` element and then setting, for example `UDS_EXAMPLE='${EXAMPLE}'`, which then just substitutes `${EXAMPLE}` in the uds-bundle.yaml with `${EXAMPLE}`. The same process works for `--set` or specifying the value to be substituted in `uds-config.yaml`, or even as a values.yaml passed as a valuesFile to the UDS package. This feels a bit clunky though, seeing as the name of the environment variable isn't likely to change all that often, so hardcoding it as a value is more clear.

### Additional context

Add any other context or screenshots about the feature request here.