The build stage ran natively on $BUILDPLATFORM and make never received a
target arch, so both manifest variants shipped the same amd64 binary
(#379). Pass TARGETOS/TARGETARCH into the build with CGO_ENABLED=0 and
verify each platform variant runs 'pmg version' after push.
Fixes#379
Claude-Session: https://claude.ai/code/session_01E8yrxjqVw7454zByb8kxvD
Co-authored-by: Claude <noreply@anthropic.com>