Support rust.channel = "auto-detect"

[pietroalbini]

As discussed in Zulip, this PR adds the new "auto-detect" value for rust.channel, to load the channel name from src/ci/channel.

Note that in a previous iteration of this PR the value was "ci" instead of "auto-detect".

[rustbot]

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[rustbot]

This PR modifies src/bootstrap/src/core/config.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

This PR modifies config.example.toml.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

[Kobzol]

Two (non-blocking) nits:

• It seems a bit weird to me to call the option "ci" when it is enabled by default for tarball sources, which are not really used on CI 😆 Maybe something like "in-tree", "from-disk" or "from-file" would be more obvious? Or just spell out "src/ci/channel"?
• If we do decide to use ci, I would prefer for the name to be more magical, to make it super obvious that it's not just another channel name (ci sounds like a reasonable channel name, actually!), e.g. by prefixing it with :, so :ci.

[bors]

☔ The latest upstream changes (presumably #137215) made this pull request unmergeable. Please resolve the merge conflicts.

[pietroalbini]

Updated the PR. Ended up choosing auto-detect as the value, since it seems more appropriate.

[Kobzol]

This is a great name. Thanks!

@bors r+

[bors]

📌 Commit 1ccff0e has been approved by Kobzol

It is now in the queue for this repository.

[bors]

🌲 The tree is currently closed for pull requests below priority 100. This pull request will be tested once the tree is reopened.

[Kobzol]

@bors rollup