migrate back off of nix-community NS (namespace) builders to GHA free runners

[colemickens]

According to the Determinate Systems blog 1, KVM can be access from free GitHub Runners.

We should probably migrate back to them from NS. For one, it would let us avoid the details of #22, return resources to nix-community for other jobs, and let us be a little cavalier about updating frequently.

Of course, we still need to return to seeing why the test doesn't actually work the way we expect.t