nixos/httpd: Use extensions from php package

After the recent rewrite, enabled extensions are passed to php programs
through an extra ini file by a wrapper. Since httpd uses shared module
instead of program, the wrapper did not affect it and no extensions
other than built-ins were loaded.

To fix this, we are passing the extension config another way – by adding it
to the service's generated config.

For now we are hardcoding the path to the ini file. It would be nice to add
the path to the passthru and use that once the PHP expression settles down.