CI: split lints + tests into seperate workflows

[rukai]

This will have a few benefits:

• Make CI "out of disk space" issues less frequent - linting and testing both produce a lot of shared artifacts but they also produce a lot of large unshared artifacts, by splitting them up across separate instances we can give each more disk space to work with. This benefit was my main motivation as I hit this issue again today.
• Make CI cached runs complete 3 mins faster - linting has become slow recently, taking ~3mins on a cached run due to having to test every combination of compile time feature. - A very nice incidental win.
• A linting failure does not prevent seeing test results.

Downsides are some more duplication in our CI configuration.