Add support for `internal` channels

Could we add support for `internal` channels? For example:

```
* curl -fsS https://dl.brave.com/install.sh | CHANNEL=nightly-internal sh
* curl -fsS https://dl.brave.com/install.sh | CHANNEL=beta-internal sh
* curl -fsS https://dl.brave.com/install.sh | CHANNEL=release-internal sh
```

This would help QA immensely, specifically the `release` channel for RC builds. For example, if you wanted to quickly check the latest RC, rather than going into https://github.com/brave/brave-browser/releases and downloading the needed `deb`, just specify `release-internal` on a clean VM and install it quickly.

CCing @fmarier @wknapik @mihaiplesa

Start with the install.sh entry point and trace how the CHANNEL value selects an installation source. Check the existing channel handling and verify the three internal channel examples on a clean VM; done means each requested channel installs the intended build, especially release-internal for RC builds.

Written by the indexing model from the issue text.