Add post-release validation workflow

[X] This contribution adheres to CONTRIBUTING.md.
[ ] ~I've updated CHANGELOG.md if applicable.~
[ ] ~I've added tests applicable for this pull request~

What does this Pull Request accomplish?

Add a workflow to test installing the latest released nimi-python modules from PyPI and running examples. It will trigger when a release is published on the repo.

Notes:

To keep things simple, I'm not using tox-system_tests.ini and I'm only running on one version of Python.
I'm using the built-in venv to provide a clean environment for every run. virtualenv is superior, but for a single Python version, venv should be good enough.
Only users with write access should be able to manually trigger the workflow

Why?

Our internal release template mentions installing modules from PyPI and running examples after publishing a release, to validate it. This also requires installing every driver. The whole thing is tedious; we should just have a workflow.

List issues fixed by this Pull Request below, if any.

None

What testing has been done?

Implemented a temporary tweak to trigger the workflow from this PR for the 1.4.8 release. All tests passed.

Running off a manual trigger or release event remains untested.

ni / nimi-python