Hide download button on blob viewer

What does this MR do and why?

When a user has read code only access via a custom role, hide the download button on the blob viewer.

Issue: #385935 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

As admin:

1. Create a group, a project within that group and apply the Ultimate license to the group
2. If SaaS mode is:
   • off: Visit http://localhost:3000/admin/application_settings/roles_and_permissions and create a custom role with the Manage deploy tokens permission enabled, based on Guest access
   • on: Visit http://localhost:3000/groups/${new_group}/-/settings/roles_and_permissions and create a custom role with the Manage deploy tokens permission enabled, based on Guest access
3. Invite a user to the project and assign the new custom role to the user

As project member with the custom role:

1. Visit http://localhost:3000/${new_group}/${new_project}/-/blob/main/README.md and verify the download button is not visible.