The `ops-tracing` release from this branch
[failed](https://github.com/canonical/operator/actions/runs/21892179689/job/63200193863)
when running `pypa/gh-action-pypi-publish@release/v1` with the following
error:
```
Traceback (most recent call last):
  File "/app/print-pkg-names.py", line 30, in <module>
    pkg_name for file_path in packages_dir.iterdir() if
                              ~~~~~~~~~~~~~~~~~~~~^^
  File "/usr/local/lib/python3.13/pathlib/_local.py", line 575, in iterdir
    with os.scandir(root_dir) as scandir_it:
         ~~~~~~~~~~^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: '/github/workspace/tracing/dist'
Warning:  It looks like there are no Python distribution packages to publish in the directory './tracing/dist/'. Please verify that they are in place should you face this problem.
ERROR    InvalidDistribution: Cannot find file (or expand pattern):             
         './tracing/dist/*'   
```
It looks like there were some shenanigans going on with where `uv build`
was putting the built files, as we have a comment linking to
https://github.com/astral-sh/uv/issues/13914 in the build step.

The built package ends up at
`/home/runner/work/operator/operator/dist/ops_tracing-2.23.2.tar.gz`, so
this PR adjusts where `gh-action-pypi-publish` should look for the files
to publish, from `./tracing/dist/` to `./dist/`.

Because we need to publish just `ops-tracing` now, this PR also adds a
`workflow_dispatch` trigger to the workflow. It can be removed in a
follow up PR.