Enhancement: `bst shell --builddir` automatically picking the newest available builddir

If you want to open a shell into a failed build, it's very often the newest builddir that interests you. Currently I have to do ls -lrt /mnt/build/cache/build/ and then manually pick the latest builddir, but BuildStream could do this for me. Of course the option to look in other builddirs is useful at times and shouldn't be removed.