Update ENV KVP tests to use `nextest` instead of `serial_test`

Not a change you should make in this PR, but I recently hit a case on something I was working on that required writing to environment variables and what I did was switch to [nextest](https://nexte.st/). It has several nice properties, including each test is run in its own process, and there's even a nice section in their docs [that goes over the safety of altering environment variables](https://nexte.st/docs/configuration/env-vars/#altering-the-environment-within-tests). It also happens to have a lot of other nice features like enforcing timeouts on slow/deadlocked tests, for example.

Anyway, I think this looks good and depending on `serial_test` is fine, but `nextest` might provide a more pleasant experience.

_Originally posted by @jeremycline in https://github.com/Azure/azure-init/pull/225#pullrequestreview-3159654720_