fix: restore cross-platform binary builds

backend/internal/bootstrap/systemd_socket_linux.go

@@ -0,0 +1,27 @@
+//go:build linux
+
+package bootstrap
+
+import (
+	"errors"
+	"fmt"
+
+	"github.com/coreos/go-systemd/activation"
+)
+
+func systemdSocket() (*socket, error) {
+	listeners, err := activation.Listeners()
+	if err != nil {
+		return nil, fmt.Errorf("failed to receive socket from systemd: %w", err)
+	}
+
+	if len(listeners) == 0 {
+		return nil, errors.New("did not receive any sockets from systemd")
+	}
+
+	if len(listeners) > 1 {
+		return nil, errors.New("received too many sockets from systemd")
+	}
+
+	return &socket{"(systemd)", listeners[0]}, nil
+}