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.