### Feature description

With the `_quarto.yml` file, different profiles can be created for, e.g., `proofing` and `production` versions of the same document.

This is useful for parallel compilation, since builds of the two versions are independent.

However, the `_variables.yml` file does not support the profile mechanism, meaning that if you have different variables for `proofing` and `production`, you still need to sequential compile runs.

Proposed solution: make `_variables.yml` behave the same way as `_quarto.yml` w.r.t. the profile.

### Steps to reproduce

_No response_

### Expected behavior

_No response_

### Actual behavior

_No response_

### Your environment

_No response_

### Quarto check output

_No response_
2023-06-16T21:46:44Z
stefanv
enhancement
open
`_variables.yml` does not support profiles
https://github.com/quarto-dev/quarto-cli/issues/5954