Build Cache!

hey @ax3l I have the workflow ready to deploy to the build cache here! You can see the PR against my repo here: https://github.com/researchapps/openPMD-api/pull/1 and the workflow running here: https://github.com/researchapps/openPMD-api/runs/4396635238?check_suite_focus=true

Here are my suggestions for moving forward!
- you'll need a PAT in the repository, DEPLOY_USER and DEPLOY_TOKEN but only on merge or scheduled run (not pull request) to deploy to GitHub packages (so the only permission we need is for packages). What I do typically is create another email for a scoped bot, and add him to the repo with the token.
- Once you have the PAT, I'll open the PR and we can review! Technically you don't need the token to build and test (e.g., see my PR) but it might be useful. I'm actually thinking we should set deploy = True for the PR and then allow it to deploy, then we can discuss what might be required in spack to make it work. E.g., we need to put metadata somewhere - either container labels or elsewhere. The buildcache workflow step will deploy the metadata to GitHub pages, but that may not be what you want.

Let me know your thoughts! When we are ready i'll open the PR!