This is a way to run `npm install` on the compat test fixtures without the node_modules at the root of the repo interfering with the node's module resolution (and that of parcel/webpack/etc).
It's hacky because ideally we'd just put each test in its own docker container for simplicity. We tried that in the private repo, but the complexity is not worth the benefit.
* Break the CI steps into separate jobs so they run in parallel and are easier to distinguish
* Cache yarn more aggressively
* Re-use actions in different jobs