fix(nix): pin cgal to 6.0.2 to unbreak sfcgal/postgis

[brainrake]

Summary

• chore: update flake.lock (monthly) #2328's nixpkgs bump (25.11pre -> 26.05pre) brings CGAL 6.1, which removes CGAL::Polygon_mesh_processing::surface_intersection. nixpkgs' sfcgal is still pinned to 2.2.0, which uses that symbol, so sfcgal fails to build and cascades into postgis.
• Upstream SFCGAL 2.3.0 adds CGAL 6.1/6.2 support, but nixpkgs hasn't bumped sfcgal to it yet.
• This adds an overlay pinning cgal back to 6.0.2 (last version sfcgal 2.2.0 builds against) until nixpkgs bumps sfcgal.
• Based on chore: update flake.lock (monthly) #2328 since the break only manifests once nixpkgs is actually bumped.

Test plan

• pkgs.cgal.version resolves to 6.0.2 through the flake's actual overlays.default
• pkgs.sfcgal builds successfully through the same overlay path on the bumped nixpkgs

[brainrake]

Rebasing this off develop instead — see the replacement PR.

[blacksmith-sh]

Blacksmith runners detected OOM events on the following jobs:

Job	Details
Check Docker Image Changes	View Job