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.

flake.nix

@@ -50,6 +50,7 @@
         pathExists
         ;
 
+      # Load private flake inputs if available
       loadDevFlake =
         path:
         let
@@ -60,7 +61,13 @@
 
       devFlake = builtins.tryEval (loadDevFlake ./devFlake/private);
 
-      privateInputs = if devFlake.success then devFlake.value.inputs else { };
+      privateInputs =
+        if pathExists ./.skip-private-inputs then
+          { }
+        else if devFlake.success then
+          devFlake.value.inputs
+        else
+          { };
     in
     flake-parts.lib.mkFlake { inherit inputs; } (
       { ... }: