Fix handling of non-existing repos during registry phase 2 migration

The fix introduced in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/83746 has
one problem, it does not set the `migration_import_done_at`
attribute on the ContainerRepository object when the import
is marked as complete. This fixes that.

app/models/container_repository.rb

@@ -174,7 +174,7 @@ class ContainerRepository < ApplicationRecord
       end
     end
 
-    before_transition %i[importing import_aborted] => :import_done do |container_repository|
+    before_transition any => :import_done do |container_repository|
       container_repository.migration_import_done_at = Time.zone.now
     end
 

spec/models/container_repository_spec.rb

@@ -139,7 +139,8 @@ RSpec.describe ContainerRepository, :aggregate_failures do
 
           expect { subject }
             .to change { repository.reload.migration_state }.to('import_done')
-            .and change { repository.reload.migration_skipped_reason }.to('not_found')
+            .and change { repository.migration_skipped_reason }.to('not_found')
+            .and change { repository.migration_import_done_at }.from(nil)
         end
       end
     end