WIP: Start every "before-build" with a clean slate

We need to start every "before-build" with an empty install directory,
otherwise the repaired wheel will contain the wrong boost libraries.
(It looks like `CMake` gets confused and picks the first matching
`libboost_python*` and `libboost_numpy*` library files, which may be
from a previous build.)

cibuildwheel/before_build.sh

@@ -20,6 +20,7 @@ function build_boost_python
   nproc=$(python -c 'import multiprocessing as mp; print(mp.cpu_count())')
   inc_dir=$(python -c 'import sysconfig as sc; print(sc.get_path("include"))')
   cd "${BOOST_BUILD_DIR}/boost"
+  rm -rf "${BOOST_INSTALL_DIR}"
   ./bootstrap.sh --prefix="${BOOST_INSTALL_DIR}" \
     --with-libraries=python \
     --with-toolset=gcc