lang/php81: Fix fetch

Upstream has re-rolled the dist for this version. This only affects
lang/php81 and not other versions of php8*. For more details please see
the following:

https://news-web.php.net/php.announce/349

Just to be confirmed that noone is using an older version as the CDN
caches might still hold the older versions bumping the PORTREVISION.

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