When re-using previously downloaded tarballs, spack copies from
`var/spack/stage/PACKAGE-VERSION-HASH/downloaded_file` to
`var/spack/cache/PACKAGE/downloaded_file`. This fails if the source is owned by
a different user (`shutil.copy` tries to retain all meta data including file
ownership). Change to a non-meta-data copy function (`shutil.copyfile`).
efadc0e299