Create separate roles for develop vs. release S3 privileges

[zackgalbreath]

The motivation behind this issue is the fact that our develop and releases mirrors both live within the s3://spack-binaries bucket.

If we use separate roles to modify these mirrors we can prevent bugs that accidentally delete release binaries when pruning the develop mirror (for example).

Per-prefix permissions are something that AWS supports. Here's an example of what this configuration might look like:

{
    "Sid": "DevelopMirrorWrite",
    "Effect": "Allow",
    "Action": [ 
        "s3:PutObject",
        "s3:GetObject",
        "s3:DeleteObject",
        "s3:ListObjectsV2"
    ],
    "Resource": "arn:aws:s3:::spack-binaries/develop/*"
}