svn loader: FileNotFoundError while creating a directory

https://sentry.softwareheritage.org/organizations/swh/issues/105236/?referrer=phabricator_plugin

FileNotFoundError: [Errno 2] No such file or directory: '/tmp/swh.loader.svn.0g8bqfec-1539429/tmpxz9z024c/tmpo96ke7rq'
(3 additional frame(s) were not displayed)
...
  File "swh/loader/svn/replay.py", line 995, in compute_objects
    self.replay(rev, low_water_mark)
  File "swh/loader/svn/replay.py", line 977, in replay
    self.conn.replay(rev, low_water_mark, self.editor)
  File "swh/loader/svn/replay.py", line 645, in close
    remove_target_path=i == 0,
  File "swh/loader/svn/replay.py", line 718, in process_external
    tempfile.mkdtemp(dir=self.editor.externals_cache_dir)
  File "tempfile.py", line 505, in mkdtemp
    _os.mkdir(file, 0o700)

[Errno 2] No such file or directory: '/tmp/swh.loader.svn.0g8bqfec-1539429/tmpxz9z024c/tmpo96ke7rq'

while loading https://svn.code.sf.net/p/z3ext/code

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Migrated from T4697 (view on Phabricator)