www/phpbb3: Fix fetch

download.phpbb.com started rejecting requests carrying the libfetch user
agent with HTTP 403. This breaks fetching the distfile everywhere, the
package building cluster included.

The distfile itself is unchanged and its recorded checksum still
matches, and there is no newer upstream release. Work around the block by
passing --user-agent to fetch.

Sponsored by: Netzkommune GmbH