Update files.py

[JoranAngevaare]

What is the problem / what does the code in this PR do
Solves a very hairy bug I induced here XENONnT/straxen#257

Can you briefly describe how it works?
So the eventbuilders are so freaking fast that we run into this issue. The metadata is still in a temp folder making the RunDBs frontend live hard.

Traceback

Traceback (most recent call last):
  File "/home/xedaq/software/straxen/bin/bootstrax", line 1142, in run_strax
    st_make()
  File "/home/xedaq/software/straxen/bin/bootstrax", line 1122, in st_make
    max_workers=process_mode['cores'])
  File "/home/xedaq/software/strax/strax/context.py", line 1085, in make
    save=save, max_workers=max_workers, **kwargs):
  File "/home/xedaq/software/strax/strax/context.py", line 901, in get_iter
    chunk_number=_chunk_number)
  File "/home/xedaq/software/strax/strax/context.py", line 745, in get_components
    check_cache(d)
  File "/home/xedaq/software/strax/strax/context.py", line 665, in check_cache
    check_cache(dep_d)
  File "/home/xedaq/software/strax/strax/context.py", line 603, in check_cache
    time_range=time_range)
  File "/home/xedaq/software/strax/strax/context.py", line 550, in _get_partial_loader_for
    sf.find(key, **self._find_options)
  File "/home/xedaq/software/strax/strax/storage/common.py", line 239, in find
    meta = self._get_backend(backend_name).get_metadata(backend_key)
  File "/home/xedaq/software/strax/strax/storage/files.py", line 215, in get_metadata
    raise strax.DataCorrupted(f"Data in {dirname} has no metadata")
strax.storage.common.DataCorrupted: Data in /data/xenonnt_processed/009655-lone_hits-nagx3zzuiv has no metadata

[WenzDaniel]

Looks fine