hosts/common/nix: migrate the nixpkgs-overlay integration point (part 2)

hosts/common/default.nix

@@ -9,7 +9,7 @@
     ./ids.nix
     ./machine-id.nix
     ./net
-    ./nix
+    ./nix.nix
     ./persist.nix
     ./polyunfill.nix
     ./programs

hosts/common/nix.nix

@@ -70,7 +70,7 @@
   # ensure new deployments have a source of this repo with which they can bootstrap.
   # this however changes on every commit and can be slow to copy for e.g. `moby`.
   environment.etc."nixos" = lib.mkIf (config.sane.maxBuildCost >= 3) {
-    source = ../../..;
+    source = ../..;
   };
   environment.etc."nix/registry.json" = lib.mkIf (config.sane.maxBuildCost < 3) {
     enable = false;

hosts/common/nix/overlay/default.nix

@@ -1,4 +0,0 @@
-# XXX: NIX_PATH=...:nixpkgs-overlays=... will import every overlay in the directory
-# so we prefer to give it a directory with just this *one* overlay, otherwise it imports conflicting overlays
-# and gets stuck in a loop until it OOMs
-import ../../../../overlays/all.nix