Spack currently has support for binary packaging, but we don't have a public mirror for source or binaries. Implement a public mirror and keep it current with sources (binaries are handled in #7116).

• stand up an S3 instance that Spack can use to house sources and binaries for builds
• In Spack's default settings (etc/spack/defaults), set this S3 instance as a fallback if package sites go down
• New source tarballs to Spack's mirror after they are merged to develop
  • require merge before add so that not just any PR adds to the mirror