flake: fix privateInputs loading in nix store contexts

When clan-core is fetched via fetchgit (e.g. in tests), the devFlake/private
directory exists but cannot be loaded as a flake. This causes errors when
building test machines.

Fix by:
1. Adding a .skip-private-inputs marker file in clan-core-for-checks to
   explicitly disable private inputs in test contexts
2. Checking for this marker file before attempting to load private inputs
3. Keeping the original tryEval approach as a fallback for compatibility

This ensures tests can run without errors while preserving the ability to
load private inputs in development environments.

checks/flake-module.nix

@@ -158,8 +158,11 @@ in
 
           clan-core-for-checks = pkgs.runCommand "clan-core-for-checks" { } ''
             cp -r ${pkgs.callPackage ./clan-core-for-checks.nix { }} $out
-            chmod +w $out/flake.lock
+            chmod -R +w $out
             cp ${../flake.lock} $out/flake.lock
+
+            # Create marker file to disable private flake loading in tests
+            touch $out/.skip-private-inputs
           '';
         };
       packages = lib.optionalAttrs (pkgs.stdenv.isLinux) {