fix buildcache create for environments with uninstalled root specs (#17859)
fix buildcache create for environments with uninstalled root specs (#17859)

* Handle uninstalled rootspecs in buildcache

- Do not parse specs / find matching specs when in an environment and no
package string is provided
- Error only when a spec.yaml or spec string are not installed. In an
environment it is fine when the root spec does not exist.
- When iterating through the matched specs, simply skip uninstalled
packages
iulian787/spack,iulian787/spack,LLNL/spack,iulian787/spack,LLNL/spack,LLNL/spack,LLNL/spack,iulian787/spack,iulian787/spack,LLNL/spack