Document new environment variables. (vercel#8620)

### Description

<!--
  ✍️ Write a short summary of your work.
  If necessary, include relevant screenshots.
-->

### Testing Instructions

<!--
  Give a quick description of steps to test your changes.
-->

docs/repo-docs/reference/system-environment-variables.mdx

@@ -15,7 +15,9 @@ System environment variables are always overridden by flag values provided direc
 | `TURBO_BINARY_PATH`                | Manually set the path to the `turbo` binary. By default, `turbo` will automatically discover the binary so you should only use this in rare circumstances.                                                                                    |
 | `TURBO_CACHE_DIR`                  | Sets the cache directory, similar to using [`--cache-dir`](/repo/docs/reference/run#--cache-dir-path) flag                                                                                                                                    |
 | `TURBO_CI_VENDOR_ENV_KEY`          | Set a prefix for environment variables that you want **excluded** from [Framework Inference](/repo/docs/crafting-your-repository/using-environment-variables#framework-inference).                                                            |
+| `TURBO_DOWNLOAD_LOCAL_ENABLED`     | Enables global `turbo` to install the correct local version if one is not found.                                                                                                                                                              |
 | `TURBO_FORCE`                      | Always force all tasks to run in full, opting out of all caching.                                                                                                                                                                             |
+| `TURBO_GLOBAL_WARNING_DISABLED`    | Disable warning when global `turbo` cannot find a local version to use.                                                                                                                                                                       |
 | `TURBO_LOG_ORDER`                  | Set the [log order](/repo/docs/reference/run#--log-order-option). Allowed values are `grouped` and `default`.                                                                                                                                 |
 | `TURBO_LOGIN`                      | Set the URL used to log in to [Remote Cache](/repo/docs/core-concepts/remote-caching).                                                                                                                                                        |
 | `TURBO_NO_UPDATE_NOTIFIER`         | Remove the update notifier that appears when a new version of `turbo` is available. You can also use `NO_UPDATE_NOTIFIER` per ecosystem convention.                                                                                           |