Fix iowrapper warning

[jorgensd]

Avoid getting:

  /root/shared/ffcx/ffcx/codegeneration/jit.py:69: ResourceWarning: unclosed file <_io.TextIOWrapper name='/root/.cache/fenics/libffcx_elements_3c34d6e913e7ee82cb1562136383f18dd61b39f3.c' mode='x' encoding='UTF-8'>
    open(c_filename, "x")
  Enable tracemalloc to get traceback where the object was allocated.
  See https://docs.pytest.org/en/stable/how-to/capture-warnings.html#resource-warnings for more info.

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html

when creating new files.
Ref: FEniCS/dolfinx#2773 (comment)

[coveralls]

coverage: 80.657% (+0.004%) from 80.653%
when pulling 4d72522 on dokken/remove-io-wrapper-warning
into 7c7530c on main.

[chrisrichardson]

It should be fine.