CLI: write to the output file, not the Path object

[Julian]

I think #432 broke output when there aren't any vulns there are skips (in my case because I'm testing trunk) -- at least here, CI jobs are failing with e.g.:

format-audit: commands[1] /home/runner/work/jsonschema/jsonschema/.tox/format-audit/tmp> /home/runner/work/jsonschema/jsonschema/.tox/format-audit/bin/python -m pip_audit
No known vulnerabilities found
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/home/runner/work/jsonschema/jsonschema/.tox/format-audit/lib/python3.11/site-packages/pip_audit/__main__.py", line 8, in <module>
    audit()
  File "/home/runner/work/jsonschema/jsonschema/.tox/format-audit/lib/python3.11/site-packages/pip_audit/_cli.py", line 543, in audit
    print(formatter.format(result, fixes), file=args.output)
AttributeError: 'PosixPath' object has no attribute 'write'. Did you mean: 'drive'?
format-audit: exit 1 (2.26 seconds) /home/runner/work/jsonschema/jsonschema/.tox/format-audit/tmp> /home/runner/work/jsonschema/jsonschema/.tox/format-audit/bin/python -m pip_audit pid=1812
  format-audit: FAIL code 1 (21.72=setup[14.21]+cmd[5.25,2.26] seconds)
  evaluation failed :( (21.86 seconds)
Error: Process completed with exit code 1.

(from https://github.com/python-jsonschema/jsonschema/actions/runs/3699379673/jobs/6266694002#step:8:70)

This seems to fix things (though I still may be getting a ResourceWarning here...)

[tetsuo-cpp]

@Julian Thanks for contributing this fix!

[tetsuo-cpp]

@woodruffw I won't put anything in the changelog for this fix since it hasn't gone out in a release yet. I'll also follow up with a unit test to cover this code in test_cli.py.

Edit: Oops, I just realised this went out in a release. Let's cut a new one now.