Revert to use Mounter method to check existence

See: https://gitlab.com/gitlab-org/gitlab-ce/commit/6280fd3777920670ee42111fddf29576cbf85988#note_14766241

We wanted to avoid accesses to the file system, because it seems reasonable that if the mounter column has some content it’s because there are an associate file. This is an speed boost when you access to a bunch of build instances to show their information.

The problem is that solution doesn’t work if you uses that method to detect the thing and you’re changing the uploaded file on instances that had that column empty. Until you don’t persist the instance the database column will be empty so we can have false negatives

CHANGELOG

@@ -14,6 +14,7 @@ v 8.13.0 (unreleased)
   - Only update issuable labels if they have been changed
   - Revoke button in Applications Settings underlines on hover.
   - Fix Long commit messages overflow viewport in file tree
+  - Revert avoid touching file system on Build#artifacts?
   - Update ruby-prof to 0.16.2. !6026 (Elan Ruusamäe)
   - Add organization field to user profile
   - Fix resolved discussion display in side-by-side diff view !6575

app/models/ci/build.rb

@@ -373,7 +373,7 @@ module Ci
     end
 
     def artifacts?
-      !artifacts_expired? && self[:artifacts_file].present?
+      !artifacts_expired? && artifacts_file.exists?
     end
 
     def artifacts_metadata?