Set `download-ci-llvm = "if-available"` by default when `channel = "dev"`

See https://github.com/rust-lang/compiler-team/issues/566.
The motivation for changing the default is to avoid downloading and building LLVM when someone runs `x build` before running `x setup`.
The motivation for only doing it on `channel = "dev"` is to avoid breaking distros or users installing from source. It works because `dev` is also the default channel.

The diff looks larger than it is; most of it is moving the `llvm` branch below the `rust` so `config.channel` is set.

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