ramus, tangra, flake: apply php backport overlay

Covers every PHP consumer: think/think-backend (php83) on ramus,
wordpress pools (pkgs.php) on tangra, and the perSystem think packages.

flake.nix

@@ -100,6 +100,7 @@
               inherit system;
               overlays = [
                 inputs.self.overlays.packagesOverlay
+                inputs.self.overlays.phpPr576853Overlay # TEMPORARY, see overlays/nixpkgs-pr-576853-php.nix
               ];
               config = { };
             };