Rollup merge of #104512 - jyn514:download-ci-llvm-default, r=Mark-Simulacrum

Rollup merge of #104512 - jyn514:download-ci-llvm-default, r=Mark-Simulacrum

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.

r? `@Mark-Simulacrum` cc `@oli-obk` `@bjorn3` `@cuviper`