Changes

feat(scripts): add --brv408 option to download br v4.0.8 (#4183)

add CLI flags -brv408/--brv408 to download br v4.0.8 from tiup mirrors.
Determine os/arch mapping, fetch the tarball, extract the 'br' binary
and save it as brv4.0.8. Fail on unsupported architectures or if the
binary is not found.

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Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
 (commit: 9686b10)