Upgrade nix version in tests

[stevenengler]

Version 1.27.0 of nix changed many (but not all) functions to take AsFd types instead of RawFd (an integer). Upgrading the tests is painful where we use integer file handles everywhere.

This version of nix also changes the epoll API, so we would also need to update tests that use the old API. But it might make more sense to not use nix for these tests, and use something closer to the actual syscall API instead.

If we're going to need to update a lot of tests, it might be worth changing them to rustix or linux-api instead.