Use Python venv on OpenBSD

[kazarmy]

Your checklist for this pull request

• I've read the guidelines for contributing to this repository
• I made sure to follow the project's coding style
• I've documented or updated the documentation of every function and struct this PR changes. If not so I've explained why.
• I've added tests that prove my fix is effective or that my feature works (if possible)
• I've updated the rizin book with the relevant information (if needed)

Detailed description

This pr uses a Python venv for the OpenBSD build as recommended here. Using --break-system-packages felt too much like bad practice.

Test plan

All builds are green.

Closing issues

...

[kazarmy]

BSD builds don't seem to be triggered so closing.