xfce-build: Always pick the latest stable release

Alternatively we could also pick the latest overall release, but the downside would be that the container could jump back and forth between 4.17 dev and 4.16 maintenance releases. git describe --tags git rev-list --tags --max-count=1 --match "$NAME*"

Please note that this MR cannot be verified by CI - you have to build / test it locally.

The downside of this approach is that once we do a new major release of Xfce we have to bump the version of the container by hand...