ci: use Go cross-compilation to eliminate slow arm64 runner

Dockerfile

@@ -1,4 +1,8 @@
-FROM golang:1.21-alpine AS builder
+# builder 阶段始终运行在构建机原生平台（amd64），用 Go 交叉编译目标平台二进制
+FROM --platform=$BUILDPLATFORM golang:1.21-alpine AS builder
+
+ARG TARGETOS
+ARG TARGETARCH
 
 WORKDIR /app
 COPY go.mod go.sum ./
@@ -8,7 +12,7 @@ RUN --mount=type=cache,target=/go/pkg/mod \
 COPY . .
 RUN --mount=type=cache,target=/go/pkg/mod \
     --mount=type=cache,target=/root/.cache/go-build \
-    CGO_ENABLED=0 GOOS=linux go build -o kiro-go .
+    CGO_ENABLED=0 GOOS=${TARGETOS} GOARCH=${TARGETARCH} go build -o kiro-go .
 
 FROM alpine:latest
 RUN apk --no-cache add ca-certificates