This expose the inner channel of "%default-channels", this can help user
to override the original guix channel with any mirror more conveniently.

Before:

```
(channel
  (inherit (car %default-channels))
  (url "url/to/mirror"))
```

After:

```
(channel
  (inherit %default-channel)
  (url "https://mirror.guix.org.cn/git/guix"))
```