USE_GITHUB: Fix WRKSRC to contain the full DISTVERSION prefix/suffx when GH_TAGNAME not set.

The GH_TAGNAME-based GH_TAGNAME_EXTRACT is now always used for the new style
USE_GITHUB WRKSRC default.

This was not spotted before since all but 1 github ports were using 'v' as a
prefix, where github already stripped it. So the default GH_PROJECT-DISTVERSION
was fine. The other case was x11-fonts/sourcesanspro-ttf where
GH_TAGNAME was defined to have the full DISTVERSION prefix/suffx.

Tested against all current USE_GITHUB !GH_COMMIT ports.

PR: 199913
With hat: portmgr
Reported by: jbeich