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

[jyn514]

See rust-lang/compiler-team#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

[jyn514]

For future work, another thing we could do is change the default profile to user when the channel is anything other than dev; but I'm not adding that here because it needs more design (for one thing, we'd need a way to opt-out).

[Mark-Simulacrum]

@bors r+

[bors]

📌 Commit ac67262 has been approved by Mark-Simulacrum

It is now in the queue for this repository.

[bors]

⌛ Testing commit ac67262 with merge 4a913d5df06ed53fb3d32fcc1c42fe00acbcfc6a...

[bors]

💔 Test failed - checks-actions

[jyn514]

Cloning into 'rust-toolstate'...
[master 53e8221] (linux CI update)
 1 file changed, 1 insertion(+)
remote: Invalid username or password.
fatal: Authentication failed for 'https://github.com/rust-lang-nursery/rust-toolstate/'

Unrelated to this PR, looks like https://rust-lang.zulipchat.com/#narrow/stream/242791-t-infra/topic/Authentication.20failed.20on.20merge.20attempt

@bors retry

[rust-log-analyzer]

The job x86_64-gnu-tools failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

 * branch            master     -> FETCH_HEAD
HEAD is now at 496fb40 (windows CI update)
[master 0c3bad5] (linux CI update)
 1 file changed, 1 insertion(+)
fatal: could not read Username for 'https://github.com': No such device or address
Sleeping for 3 seconds before retrying push
 * branch            master     -> FETCH_HEAD
HEAD is now at 496fb40 (windows CI update)
[master f695733] (linux CI update)
 1 file changed, 1 insertion(+)
 1 file changed, 1 insertion(+)
fatal: could not read Username for 'https://github.com': No such device or address
Sleeping for 3 seconds before retrying push
 * branch            master     -> FETCH_HEAD
HEAD is now at 496fb40 (windows CI update)
[master e9dafae] (linux CI update)
 1 file changed, 1 insertion(+)
 1 file changed, 1 insertion(+)
fatal: could not read Username for 'https://github.com': No such device or address
Sleeping for 3 seconds before retrying push
 * branch            master     -> FETCH_HEAD
HEAD is now at 496fb40 (windows CI update)
[master e4ca701] (linux CI update)
 1 file changed, 1 insertion(+)
 1 file changed, 1 insertion(+)
fatal: could not read Username for 'https://github.com': No such device or address
Sleeping for 3 seconds before retrying push
 * branch            master     -> FETCH_HEAD
 * branch            master     -> FETCH_HEAD
thread 'main' panicked at 'Failed to update toolstate repository with new data', toolstate.rs:436:9
HEAD is now at 496fb40 (windows CI update)
Build completed unsuccessfully in 0:00:17