Try to fix #7669

The issue *seems* to be the cross jobs, which are missing the `CXXFLAGS`
needed to get rapidcheck.

PR #6538 would be really nice to resurrect which will prevent the
`configureFlags` from going out of sync between the regular build and
the cross build again.

flake.nix

@@ -653,6 +653,7 @@
  inherit system crossSystem;
  overlays = [ self.overlays.default ];
  };
+ inherit (nixpkgsCross) lib;
  in with commonDeps { pkgs = nixpkgsCross; }; nixpkgsCross.stdenv.mkDerivation {
  name = "nix-${version}";
 
@@ -665,7 +666,11 @@
  nativeBuildInputs = nativeBuildDeps;
  buildInputs = buildDeps ++ propagatedDeps;
 
- configureFlags = [ "--sysconfdir=/etc" "--disable-doc-gen" ];
+ configureFlags = [
+ "CXXFLAGS=-I${lib.getDev nixpkgsCross.rapidcheck}/extras/gtest/include"
+ "--sysconfdir=/etc"
+ "--disable-doc-gen"
+ ];
 
  enableParallelBuilding = true;