feat(nixos/whitby): Hello, World!

This adds NixOS configuration for the machine whitby.tvl.fyi.

No interesting services are configured yet, so this configuration is
quite plain.

Change-Id: I67b7c75ebd6e298719b52e6b3bd83cc3be3c45d8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/843
Tested-by: BuildkiteCI
Reviewed-by: BuildkiteCI
Reviewed-by: isomer <isomer@tvl.fyi>
Reviewed-by: lukegb <lukegb@tvl.fyi>

ops/nixos/default.nix

@@ -0,0 +1,13 @@
+# Most of the Nix expressions in this folder are NixOS modules, which
+# are not readTree compatible.
+#
+# Some things (such as system configurations) are, and we import them
+# here manually.
+#
+# TODO(tazjin): Find a more elegant solution for the whole module
+# situation.
+{ ... }@args:
+
+{
+  whitby = import ./whitby/default.nix args;
+}