move `follows` to top-level inputs

nix evaluates the  from the root flake, meaning that nested follows don't work well when this flake is imported in another flake

flake.lock

@@ -87,11 +87,9 @@
         ],
         "flake8-artiq": "flake8-artiq",
         "nixpkgs": [
-          "artiq",
           "nixpkgs"
         ],
         "sipyco": [
-          "artiq",
           "sipyco"
         ],
         "trap-dac-utils": "trap-dac-utils"
@@ -185,7 +183,15 @@
     "root": {
       "inputs": {
         "artiq": "artiq",
-        "daxpkgs": "daxpkgs"
+        "daxpkgs": "daxpkgs",
+        "nixpkgs": [
+          "artiq",
+          "nixpkgs"
+        ],
+        "sipyco": [
+          "artiq",
+          "sipyco"
+        ]
       }
     },
     "sipyco": {

flake.nix

@@ -2,19 +2,21 @@
   description = "Additional packages for ARTIQ";
 
   inputs.artiq.url = git+https://github.com/m-labs/artiq.git;
+  inputs.nixpkgs.follows = "artiq/nixpkgs";
+  inputs.sipyco.follows = "artiq/sipyco";
 
   inputs.daxpkgs = {
     url = git+https://gitlab.com/duke-artiq/dax.git;
     inputs = {
       artiqpkgs.follows = "artiq";
-      nixpkgs.follows = "artiq/nixpkgs";
-      sipyco.follows = "artiq/sipyco";
+      nixpkgs.follows = "nixpkgs";
+      sipyco.follows = "sipyco";
     };
   };
 
-  outputs = { self, artiq, daxpkgs }:
+  outputs = { self, artiq, daxpkgs, nixpkgs, sipyco }:
     let
-      pkgs = import artiq.inputs.nixpkgs { system = "x86_64-linux"; };
+      pkgs = import nixpkgs { system = "x86_64-linux"; };
       msys2 = import ./msys2 { inherit pkgs; };
 
       condaDeps = with pkgs; [ zlib xorg.libSM xorg.libICE xorg.libX11 xorg.libXau xorg.libXi xorg.libXrender libselinux libGL ];