Include flag to set different permissions during s3invsync transfer

Permissions set on files that were transfers were not optimal for read-only access across groups.

During our `s3invsync` transfer, files copied over to MIT Engaging had a default permissions of:

**File-level:**

`-rw-------+`

**Directory Level**

`drwxrws---+`

When first ssh'd onto a login node, output of `umask` is `0002`; however, these permissions do not match such an setting that you'd expect from the default umask value.

The ideal permissions for our group are:

**File-level**

`-rw-r-----+`

**Directory-level**

`drwxrws---+` (which was appropriate from the initial copy)

This GitHub issue captures the need to perhaps set temporary permissions that enable read-only access across groups/users for not just the user that transfers the files, but also others.

Perhaps a quick setting of `umask 027`, etc. could work here -- rather integrate something into `s3invsync` though upstream -- thanks all

Cc @kabilar @yarikoptic @satra @jwodder