overlays: backport nixpkgs#576853 (php 8.3.36/8.4.27/8.5.12)

Temporary overlay until the PR reaches nixpkgs-unstable. Fixes
CVE-2026-109618 and CVE-2026-109619. Becomes a no-op and warns at eval
time once the locked nixpkgs ships these versions; delete it then.

overlays/default.nix

@@ -2,6 +2,8 @@
 {
   flake.overlays = {
     packagesOverlay = import ../pkgs/overlay.nix;
+    # TEMPORARY: drop with the file once NixOS/nixpkgs#576853 reaches nixpkgs-unstable.
+    phpPr576853Overlay = import ./nixpkgs-pr-576853-php.nix;
 
     frappixFrappeOverlay = inputs.frappix.frappeOverlay.x86_64-linux;
     frappixLibsOverlay = inputs.frappix.libsOverlay.x86_64-linux;