Guests can read builds if those are public

Guests were not able to read builds/pipelines but we claimed in the docs this was possible.

However, the build details will remain hidden for guests

app/policies/ci/build_policy.rb

 module Ci
   class BuildPolicy < CommitStatusPolicy
     def rules
+      can! :read_build if @subject.project.public_builds?
+
       super
 
       # If we can't read build we should also not have that