nix/common.nix: use pattern from nix-community/flake-compat

This is still a bit confusing, as normally the flake.defaultNix attrset
gets exposed via a top-level default.nix, but at least it brings us
closer to that.

nix/common.nix

@@ -1,14 +1,17 @@
 let
   lock = builtins.fromJSON (builtins.readFile ../flake.lock);
-  flakeCompatRev = lock.nodes.flake-compat.locked.rev;
-  flakeCompat = fetchTarball {
-    url =
-      "https://github.com/edolstra/flake-compat/archive/${flakeCompatRev}.tar.gz";
-    sha256 = lock.nodes.flake-compat.locked.narHash;
+
+  inherit (lock.nodes.flake-compat.locked) owner repo rev narHash;
+
+  flake-compat = fetchTarball {
+    url = "https://github.com/${owner}/${repo}/archive/${rev}.tar.gz";
+    sha256 = narHash;
   };
-  flake = ((import flakeCompat) { src = ../.; }).defaultNix;
-in rec {
-  pkgsSrc = flake.inputs.nixpkgs;
-  cargo2nix = flake.inputs.cargo2nix;
+
+  flake = (import flake-compat { system = builtins.currentSystem; src = ../.; });
+in
+rec {
+  pkgsSrc = flake.defaultNix.inputs.nixpkgs;
+  cargo2nix = flake.defaultNix.inputs.cargo2nix;
   cargo2nixOverlay = cargo2nix.overlays.default;
 }