Bypass build_cache/index.html read when trying to download spec.yaml for concretized spec. (#14698)

* Add binary_distribution::get_spec which takes concretized spec
Add binary_distribution::try_download_specs for downloading of spec.yaml files to cache
get_spec is used by package::try_install_from_binary_cache to download only the spec.yaml
for the concretized spec if it exists.