Add some tooling to support copying stack binaries to root

[scottwittenburg]

Add some scripts to do something similar to what the spack protected-publish job currently does, but after the fact, and for an arbitrary ref. Currently these scripts:

• take an arbitrary ref and fetch the latest pipeline for it
• download artifacts from each of the pipeline generation jobs
• iterate over the downloaded spack.lock files, and for each:
  • create and activate the environment
  • use spack buildcache sync to copy all environment specs to another location

I am currently using these scripts for two purposes:

• create top-level buildcaches for recent develop snapshots for which protected-publish did not run
• evaluate proposed performance improvements to spack buildcache sync

I'm not sure these scripts will ultimately end up in one of our custom images, but we might think about how to use them to take the protected-publish job out of spack pipelines.