ci: make sure codecov is working as expected

[terriko]

• possibly related: ci: pull request aren't being reported with code coverage reports #4317

When I split up the long test job into 4 pieces, I may have messed up our codecov setup even more than I already had (see #4316 -- we already didn't it was working as expected). I could use some help figuring out what's going wrong and what needs to happen to fix it. I'm also open to trying another coverage tool or just getting reporting out of pytest or something if our current setup isn't working for us.

I'm going to mark this as a potential good first issue since it doesn't require much cve-bin-tool specific knowledge, but be aware that you may have to do a lot of reading on how codecov works. If you give it a shot and get stuck, please let us know where so the next person doesn't wind up trying the same things!

[terriko]

Note that you should be able to test a setup on a fork of cve-bin-tool (so you have whatever admin privileges are needed on the codecov.io side) and once you've got it working then make a pull request.