tests/docker: Fix _get_so_libs() for docker-binfmt-image

tests/docker: Fix _get_so_libs() for docker-binfmt-image

Fix a variable rename mistake from commit 5e33f7fead5:

  Traceback (most recent call last):
    File "./tests/docker/docker.py", line 710, in <module>
      sys.exit(main())
    File "./tests/docker/docker.py", line 706, in main
      return args.cmdobj.run(args, argv)
    File "./tests/docker/docker.py", line 489, in run
      _copy_binary_with_libs(args.include_executable,
    File "./tests/docker/docker.py", line 149, in _copy_binary_with_libs
      libs = _get_so_libs(src)
    File "./tests/docker/docker.py", line 123, in _get_so_libs
      libs.append(s.group(1))
  NameError: name 's' is not defined

Fixes: 5e33f7fead5 ("tests/docker: better handle symlinked libs")
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210119050149.516910-1-f4bug@amsat.org>
Message-Id: <20210202134001.25738-2-alex.bennee@linaro.org>