Add py38-expat package to netbsd.yml

[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

https://www.mail-archive.com/[email protected]/msg960741.html gives hope that this pr will work in fixing #3832.

Test plan

All builds are green.

Closing issues

Closes #3832.

[XVilka]

LGTM if green