os/mac/xcode: add additional CLT download link.

[MikeMcQuaid]

Sadly Apple don't seem to be providing these reliably.

Fixes #6796

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[bpzhang]

This seems to only modify the prompt information, but it does not actually solve the problem. There will still be a new version, but it is actually the latest stable version

[MikeMcQuaid]

@bpzhang This solves the problem where people do not know how to get to the latest version. I see no evidence there's a bug beyond that.