feat: avoid newline separated cherry-picks, use arrays

To make both eicweb and GitHub happy with the list of cherry-picks in spack.sh, we need to turn this into a space-separated list to pass as a build argument, not a newline-separate list that only works in eicweb.

This PR turns the lists in spack.sh into arrays, and passes them as "${SPACK_CHERRYPICKS[@]}" to the build args. Inside the dockerfile, they are now treated as space-separated, and all spaces are replaced with | for an extended regular expression.