Disable ubuntu-24.04-arm jobs for now

They don't start at all and they get cancelled.
https://github.com/ruby/ruby/actions/runs/17476949351/job/49639049451

To unblock PR merges, let's disable it until it's fixed.

Author: k0kubun

.github/workflows/ubuntu.yml

@@ -39,8 +39,9 @@ jobs:
           - test_task: test-bundled-gems
           - test_task: check
             os: ubuntu-24.04
-          - test_task: check
-            os: ubuntu-24.04-arm
+          # ubuntu-24.04-arm jobs don't start on ruby/ruby as of 2025-09-04
+          #- test_task: check
+          #  os: ubuntu-24.04-arm
       fail-fast: false
 
     env: