PHP ZTS build in nixpkgs

In 2022, nixpkgs changed the default PHP build from ZTS (Zend Thread Safety) to NTS (non-thread-safe) as discussed in NixOS/nixpkgs#154774. Since then, nixpkgs has provided only the NTS build; there is currently no separate PHP ZTS package or variant. A couple of things has changed since then. First, PHP upstream is planning to deprecate NTS builds and make ZTS the default starting from PHP 8.6, according to the PHP internals discussion. Second, the performance overhead of ZTS has been greatly ...