Revert "CI: Re-enable Ubuntu arm with CAPI check"

This reverts commit 02fd628.

It stopped working again.

Author: k0kubun

.github/workflows/ubuntu.yml

@@ -40,9 +40,9 @@ jobs:
           - test_task: check
             os: ubuntu-24.04
             extra_checks: [capi]
-          - test_task: check
-            os: ubuntu-24.04-arm
-            extra_checks: [capi]
+          # ubuntu-24.04-arm jobs don't start on ruby/ruby as of 2025-10-29
+          #- test_task: check
+          #  os: ubuntu-24.04-arm
       fail-fast: false
 
     env: &make-env