pmb.build._package: update isl workaround

The plan was to drop this workaround, once alpine releases a new gcc
version, and we regenerate our cross compilers against that version
_and_ drop options="!tracedeps", as latest pmbootstrap aportgen would
do. However !tracedeps wasn't dropped in the latest upgrade. This was
too easy to miss, I'll add a comment to the gcc-cross APKBUILDs so we
see it next time. Let's just update the workaround for this version.

Related: pmaports#1732 (closed)