A new field is introduced to grant.Permission in storj.io/common. Having
a direct cast here leads to compilation problems when bumping
storj.io/uplink to the latest storj.io/common. Avoiding the direct cast
resolves the issue.
Context: https://github.com/storj/storj/issues/6249
Change-Id: I3b9bc14ebcce8e192e218c621b996300753b8de4