php: add buildEnv function for additional config on the CLI SAPI

Initially discussed in #55460.

This patch adds a `buildEnv` function to `php` that has the
following features:

* `php.buildEnv { extraConfig = /* ... */; }` to specify custom
  `php.ini` args for the CLI.

* `php.buildEnv { exts = phpPackages: [phpPackages.apcu] }` to
  create a PHP interpreter for the CLI with the `apcu` extension.

pkgs/top-level/php-packages.nix

@@ -664,6 +664,8 @@ let
 
     doCheck = true;
     checkTarget = "test";
+
+    zendExtension = true;
   };
 
   yaml = buildPecl {