Replace git clone spack-packages with spack repo add command

[wdconinc]

Briefly, what does this PR introduce?

This PR modifies the way the spack-package repo is added to the container. Instead of putting the repo in the container with a specific git clone, we now use the new spack repo add command which retains the information that this is a repository and which then supports spack repo update. When spack only knows of a directory (previous situation), there is nothing to update.

What kind of change does this PR introduce?

• Bug fix (issue Cannot update Spack builtin package repo when running container interactively #29)
• New feature (issue #__)
• Documentation update
• Other: __

Please check if this PR fulfills the following:

• Tests for the changes have been added
• Documentation has been added / updated
• Changes have been communicated to collaborators

Does this PR introduce breaking changes? What changes might users need to make to their code?

No.

Does this PR change default behavior?

No.

[wdconinc]

This PR turns out to be harder than anticipated. When using spack repo add you lose the .git dir and its history. Since we apply cherry-picks, we need that history.

This has been passed to the spack team as a suggestion.

[wdconinc]

We may be able to do this with a spack repo update --commit for every commit, to fill the history in the shallow commit...