fix(flake): switch overlay to callPackage

This ensures it can be built for any architecture (supported by Rust).

default.nix

@@ -0,0 +1,37 @@
+{
+  pkgs ?
+    (builtins.getFlake (builtins.toString ./.)).inputs.nixpkgs.legacyPackages.${builtins.currentSystem},
+  lib ? pkgs.lib,
+  crane ? (builtins.getFlake (builtins.toString ./.)).inputs.crane,
+  pkgSrc ? ./.,
+  mold ? pkgs.mold,
+}:
+let
+  craneLib = crane.mkLib pkgs;
+  src = craneLib.cleanCargoSource pkgSrc;
+
+  commonArgs = {
+    inherit src;
+    strictDeps = true;
+
+    doCheck = false; # tests will be run in the `checks` derivation
+    NEXTEST_HIDE_PROGRESS_BAR = 1;
+    NEXTEST_FAILURE_OUTPUT = "immediate-final";
+
+    nativeBuildInputs = [ mold ];
+
+    meta = {
+      license = lib.licenses.mit;
+      homepage = "https://github.com/jalil-salame/webnsupdate";
+      mainProgram = "webnsupdate";
+    };
+  };
+
+  cargoArtifacts = craneLib.buildDepsOnly commonArgs;
+in
+craneLib.buildPackage (
+  lib.mergeAttrsList [
+    commonArgs
+    { inherit cargoArtifacts; }
+  ]
+)