Replace old `ngx_cache_purge` module source with a maintained fork

Currently the script downloads the `ngx_cache_purge` module from [FRiCKLE](https://github.com/FRiCKLE/ngx_cache_purge), which had its last update 8 years ago. No wonder i've had problems using that module with nginx v1.22.0 (stable).

I suggest using an actively maintained fork, for example the one from [nginx-modules](https://github.com/nginx-modules/ngx_cache_purge) or from [torden](https://github.com/torden/ngx_cache_purge).

I already tried it by manually changing the source in the script from `/FRiCKLE/` to `/nginx-modules/` (on line 367) and everything works like a charm.