The runner's stale global GOROOT pointed 'go' 1.25.0 at a 1.26.4 compile tool
('compile ... does not match go tool version'), which go clean -cache could not
fix. Resolve GOROOT from the active go binary and export it for all steps, with
diagnostics, then clean the cache under that GOROOT.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>