nixos/systemd-boot-builder: make sure to print informational messages to stderr

Otherwise this gets printed to stdout when running things like
nixos-rebuild, while people rely on the toplevel store path being the only
output of such commands.

nixos/modules/system/boot/loader/systemd-boot/systemd-boot-builder.py

@@ -362,7 +362,7 @@ def install_bootloader(args: argparse.Namespace) -> None:
         available_version = available_match.group(1)
 
         if installed_version < available_version:
-            print("updating systemd-boot from %s to %s" % (installed_version, available_version))
+            print("updating systemd-boot from %s to %s" % (installed_version, available_version), file=sys.stderr)
             run(
                 [f"{SYSTEMD}/bin/bootctl", f"--esp-path={EFI_SYS_MOUNT_POINT}"]
                 + bootctl_flags