Include public signing key when publishing stacks

[scottwittenburg]

When moving the protected-publish job from spack gitlab pipelines to a dedicated cron job, I neglected to take care of publishing the signing key to the root mirror. This PR resolves that issue.

I tested this change by removing a spec from the s3://spack-binaries/v0.23.0 mirror, then building the imgae locally, and running the container to publish the v0.23.0 ref. After which, I could successfully install keys from that mirror:

$ git clone https://github.com/spack/spack.git
$ cd spack
$ git checkout v0.23.0
$ spack mirror add binaries s3://spack-binaries/v0.23.0
$ spack buildcache keys --install
==> Fetching s3://spack-binaries/v0.23.0/build_cache/_pgp/spack-public-binary-key.pub