On Cray systems that use Cray Data Virtualization Service (DVS),
symlinks across filesystems are not allowed, either due to a bug, or
because they're simply not POSIX compliant [1].
Spack's OpenSSL package defaults to `certs=system` which comes down to
symlinking `/etc/ssl` in the Spack install prefix, triggering this
problem, resulting in mysterious installation failures.
Instead of relying on system certs, we can just use
`ca-certificates-mozilla`, and if users really need system certs, then
they're probably better off marking OpenSSL entirely as external.
[1] https://github.com/glennklockwood/cray-dvs
1c0bf12e5b