squash! flake: decouple client-side tools from x86_64-linux

Override contrastPkgsStatic on darwin: use linux static packages from
runtimePkgs but keep genpolicy as a native darwin build since the CLI
embeds and runs it on the host. The full darwin contrastPkgsStatic
fails to evaluate because genpolicy's C deps (aws-lc-sys) don't build
with darwin's static toolchain.

Author: sespiros

overlays/runtimepkgs.nix

@@ -9,6 +9,15 @@ else
   {
     contrastPkgs = prev.contrastPkgs.overrideScope (
       _cFinal: cPrev: {
+
+        contrastPkgsStatic = final.runtimePkgs.contrastPkgsStatic.overrideScope (
+          _: _: {
+            kata = final.runtimePkgs.contrastPkgsStatic.kata.overrideScope (
+              _: _: { inherit (cPrev.kata) genpolicy; }
+            );
+          }
+        );
+
         kata = cPrev.kata.overrideScope (
           _: _: {
             inherit (final.runtimePkgs.kata)