fix NIX_PATH overlay interaction that was crashing nix-shell

hosts/common/default.nix

@@ -8,6 +8,7 @@
     ./ids.nix
     ./machine-id.nix
     ./net.nix
+    ./nix-path
     ./persist.nix
     ./programs
     ./secrets.nix
@@ -36,11 +37,6 @@
   nix.extraOptions = ''
     experimental-features = nix-command flakes
   '';
-  # allow `nix-shell` (and probably nix-index?) to locate our patched and custom packages
-  nix.nixPath = [
-    "nixpkgs=${pkgs.path}"
-    "nixpkgs-overlays=${../..}/overlays"
-  ];
   # hardlinks identical files in the nix store to save 25-35% disk space.
   # unclear _when_ this occurs. it's not a service.
   # does the daemon continually scan the nix store?

hosts/common/nix-path/default.nix

@@ -0,0 +1,10 @@
+{ pkgs, ... }:
+
+{
+  # allow `nix-shell` (and probably nix-index?) to locate our patched and custom packages
+  nix.nixPath = [
+    "nixpkgs=${pkgs.path}"
+    # note the import starts at repo root: this allows `./overlay/default.nix` to access the stuff at the root
+    "nixpkgs-overlays=${../../..}/hosts/common/nix-path/overlay"
+  ];
+}

hosts/common/nix-path/overlay/default.nix

@@ -0,0 +1,4 @@
+# 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