winbuild.sh: Clear old build dir
This has not been an issue on nightly builds as those delete quite a lot of stuff before even running freeciv's own build scripts.
This is more of an issue with the docker container build, which I've wanted to implement not to do too much destructive operations as people may use it for a development source tree, with their own changes present.
winbuild.sh assumes that the build directory does not exist beforehand. If it does, the build happily tries to take place the stuff already present in there.
This is different from handling of install directory of both winbuild.sh and meson-winbuild.sh, and handling of build dir of meson-winbuild.sh.