coverage: correct why the baseline action can't use covdata

The comment said the Go SDK does not ship a prebuilt covdata. That is
only true from Go 1.25, which dropped it from the distribution to reduce
download size. Go 1.24 and earlier do ship it.

Rest the reasoning on what this action is given instead: no covdata on
current SDKs, and no way to decode a meta file in process, since
go_tool_binary builds with "go build" and internal/coverage is off
limits. Also record why the choice is safe, which the comment never said
before. Both forms take their spans from one shared walk.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>