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.
0bf5619fe0