GitHub Actions: Use ldconfig -p | grep libmagic to discover Ubuntu libmagic

[cclauss]

Let's get the macOS and Ubuntu tests passing again.

test/python_magic_test.py -- test_all_cases() seems to be a work in progress. Broken pytests belong in pull requests, not on master!

@ahupp Your review, please. The repo should have ci tests that pass.

[ahupp]

I'd like to use normal shared library lookup paths as much as possible, rather than hard-coding for specific distributions. Those kind of updates have been by volume the most common changes to this package over the years. I think the issue you're seeing is due to a broken merge I did a while back that is fixed in #346.

[ahupp]

re: adding a dependency on python-magic-bin, I'm not comfortable with that. The immediate issue is that it uses the same package name as python-magic but hasn't been updated forever, and depending on the specific ordering of things in pip the import magic will give you one version or another.

But also, it has binaries built from who knows what source by who knows what person, and I think given any binary I ship (or depend on) needs to be built out of github CI from a known-good source and uploaded directly from there.