Test using Hydra with flakes

It seemed there was no self-contained end-to-end test actually doing
this?!

Among other things, this will help ensure that the switch-over to
`nix-eval-jobs` is correct.

t/jobs/flake-checks/flake.nix

@@ -0,0 +1,6 @@
+{
+  outputs = { ... }: {
+    checks =
+      import ./basic.nix;
+  };
+}