fix: dont double-apply overlays

previous behavior was that overlays were *implicitly* applied when i
imported nixpkgs, and then explicitly applied again later in the config.

for some reason i can't remove (or adjust?) the implicit application
without causing evals to hang w/o so much as any error message.

pkgs/additional/nixpkgs/default.nix

@@ -39,7 +39,13 @@ let
     url = "https://github.com/NixOS/nixpkgs/archive/${lock'.rev}.tar.gz";
     inherit (lock') sha256;
   };
-  unpatchedNixpkgs = import unpatchedSrc { inherit localSystem; };
+  commonNixpkgsArgs = {
+    inherit localSystem;
+    # reset impurities
+    config = {};
+    overlays = [];
+  };
+  unpatchedNixpkgs = import unpatchedSrc commonNixpkgsArgs;
 
   patchedSrc = unpatchedNixpkgs.applyPatches {
     name = "nixpkgs-patched-uninsane";
@@ -56,8 +62,7 @@ let
   };
 
   src = if doPatch then patchedSrc else { outPath = unpatchedSrc; };
-  args = {
-    inherit localSystem;
+  args = commonNixpkgsArgs // {
     config = {
       allowUnfree = true;  # NIXPKGS_ALLOW_UNFREE=1
       allowBroken = true;  # NIXPKGS_ALLOW_BROKEN=1