Reintroduce realpath
MR !39 (merged) did not address the case when the build cache folder is a symlink.
the code in the shell script (somewhere) requires the absolute path and not the symlink,
which was the reason behind using realpath ../mpsd-spack-cache
.
This MR brings that back and ensures that the realpath command also works on mac by running mkdir -p ../mpsd-spack-cache
before (ensuring the folder exists when calling realpath)