deskutils/zk: Update to 0.15.5

Changelog: https://github.com/zk-org/zk/blob/v0.15.5/CHANGELOG.md

Fix the build, which has been broken from the beginning, with every
vendored package erroring with:

"cannot find module providing package X import lookup disabled by -mod=vendor"

Switch to using GO_MODULE to fetch deps.

While here, update my maintainer email.

Approved by: dch (mentor)
Pull Request: https://github.com/freebsd/freebsd-ports/pull/553