Optimize CI pipelines

[kjappelbaum]

• [ ] Flake8 takes ages (14 min) - nearly as long as pytest (20 min)- probably because we use it via tox?
• [ ] Cache dependencies (building takes 8 min)

[kjappelbaum]

we could cache the tox stuff https://github.com/lsst-sqre/safir/blob/master/.github/workflows/ci.yaml