Fix OpenBLAS binary installation (#89)

The archives directory was not created. I tested this downstream and the fix worked.

_gfortran_utils.sh

 # Stripped-down version of bash utilities for use with gfortran
 
-ARCHIVE_SDIR="${ARCHIVE_SDIR:-archives}"
-
 function _mb_get_gf_lib_for_suf {
     local suffix=$1
     local prefix=$2

library_builders.sh

@@ -102,6 +102,7 @@ function build_openblas {
         brew install openblas
         brew link --force openblas
     else
+        mkdir -p $ARCHIVE_SDIR
         local plat=${1:-$PLAT}
         local tar_path=$(abspath $(_mb_get_gf_lib "openblas-${OPENBLAS_VERSION}" "$plat"))
         (cd / && tar zxf $tar_path)