avoid TypeError being raised by list_toolchains

[boegel]

This fixes a TypeError that is (sometimes) raised when running --list-toolchains:

Traceback (most recent call last):
  File "/home/runner/work/easybuild-framework/easybuild-framework/easybuild/tools/options.py", line 1353, in parse_options
    eb_go = EasyBuildOptions(usage=usage, description=description, prog=\'eb\', envvar_prefix=CONFIG_ENV_VAR_PREFIX,
  File "/home/runner/work/easybuild-framework/easybuild-framework/easybuild/tools/options.py", line 242, in __init__
    super(EasyBuildOptions, self).__init__(*args, **kwargs)
  File "/home/runner/work/easybuild-framework/easybuild-framework/easybuild/base/generaloption.py", line 924, in __init__
    self.postprocess()
  File "/home/runner/work/easybuild-framework/easybuild-framework/easybuild/tools/options.py", line 856, in postprocess
    self._postprocess_list_avail()
  File "/home/runner/work/easybuild-framework/easybuild-framework/easybuild/tools/options.py", line 1099, in _postprocess_list_avail
    msg += list_toolchains(self.options.output_format)
  File "/home/runner/work/easybuild-framework/easybuild-framework/easybuild/tools/docs.py", line 730, in list_toolchains
    tclist = sorted(zip(all_tcs_names, all_tcs))
TypeError: \'<\' not supported between instances of \'type\' and \'type\'

fixes #3469

[bartoldeman]

LGTM