ci: 按 runner 架构下载对应的 Go 工具链

runner 为 arm64,之前固定下载 amd64 版 Go 导致 Exec format error;
改为用 uname -m 检测宿主架构后下载匹配的 Go tarball

.gitea/workflows/build.yml

@@ -20,7 +20,12 @@ jobs:
             apt-get install -y --no-install-recommends git curl ca-certificates tar
           fi
           GO_VERSION=1.26.0
-          curl -fsSL "https://go.dev/dl/go${GO_VERSION}.linux-amd64.tar.gz" -o /tmp/go.tar.gz
+          case "$(uname -m)" in
+            x86_64)  HOST_ARCH=amd64 ;;
+            aarch64) HOST_ARCH=arm64 ;;
+            *) echo "unsupported host arch: $(uname -m)" >&2; exit 1 ;;
+          esac
+          curl -fsSL "https://go.dev/dl/go${GO_VERSION}.linux-${HOST_ARCH}.tar.gz" -o /tmp/go.tar.gz
           rm -rf /usr/local/go
           tar -C /usr/local -xzf /tmp/go.tar.gz
           /usr/local/go/bin/go version