math/yices: Fix distfile source specification

Before it pulled the branch called 2.7.0 not the tagged release
and only worked by accident.

Tagged release:
https://github.com/SRI-CSL/yices2/releases/tag/yices-2.7.0

Branch:
https://github.com/SRI-CSL/yices2/tree/2.7.0

make -V GH_TAGNAME
2.7.0

GH_TAGNAME should be yices-${DISTVERSION}

Reported by: diizzy (via mailing list)
Approved by: yuri@ (Mentor, implicit)

Fixes: 364ccd859655 * math/yices: Update 2.6.5 => 2.7.0, mark as PIE_UNSAFE