ci: add nightly job to sync source mirror with pkgs in builtin

[alecbcs]

Add a minimal nightly GitHub Actions CI/CD job that compares the state of the package repository to the mirrored source cache and mirrors any artifacts that are missing from the cache. (As of the time of posting there are 439 missing package versions that would be uploaded by this workflow.)

Will need to work with @kwryankrattiger and @jjnesbitt before merging to update the OIDC rules in AWS to allow this new workflow.

[scheibelp]

Other points (not requests for this PR):

• (request for later) If a patch/resource is dropped, this doesn't catch that: if we already placed the "main" version tarball in a mirror, and a resource was missed, then this job will also miss that (I see though that patches/resources generally are included, just not in this rare case). I think this can be addressed later.
• (not a request) What about wrong checksums (i.e. if someone specifies the wrong checksum for a version)? I think the behavior would be OK: this script will collect every wrong checksum, download the version (which should fail, but spack mirror generally proceeds on in this case). We'd want to clean those up though.

[scheibelp]

Request: limit this to something like 100-500 and print a warning if we go over that (tty.warn? something that prints to stderr)