Restrict flake to Linux systems and enable allowUnfree for nixosConfigurations

This project is Linux-specific (microVMs, libkrun, systemd, etc.), so
the flake now only exports packages and checks for x86_64-linux and
aarch64-linux systems. This prevents Darwin build failures.

Additionally, nixosConfigurations now set allowUnfree = true to allow
packages like zerotierone that have unfree licenses.

flake.nix

@@ -257,6 +257,7 @@
       modules = [
         self.nixosModules.default
         self.nixosModules.testvm
+        { nixpkgs.config.allowUnfree = true; }
       ];
     };
 
@@ -265,9 +266,10 @@
       modules = [
         self.nixosModules.default
         self.nixosModules.testvm
+        { nixpkgs.config.allowUnfree = true; }
       ];
     };
-  } // flake-utils.lib.eachDefaultSystem (system:
+  } // flake-utils.lib.eachSystem [ "x86_64-linux" "aarch64-linux" ] (system:
       let
         pkgs = import nixpkgs {
           inherit system;