Spack "upstream not found" warning

[wyphan]

Creating an Exawind snapshot on a freshly-cloned spack-manager makes Spack emit this warning:

==> Warning: upstream not found: /path/to/spack-manager/spack/opt/spack/.spack-db/index.json

Normally this warning is harmless, but for container builds, it makes Spack return an error code of 2, which terminates the container build process early. Regardless, the warning is actually valid, since with a fresh clone, no Spack packages have been installed yet, and such the upstream Spack database doesn't exist.

There are two ways to fix this:

• Skip setting up the upstream entirely
• Install a package in upstream before running the create snapshot script