[Cirrus] Run only if nightly [ci skip]

Recently Cirrus-CI does not start over 10 hours.  Maybe we have too
many runs.

.cirrus.yml

@@ -11,6 +11,7 @@ env:
 task:
   name: Arm64 Graviton2 / $CC
   auto_cancellation: $CIRRUS_BRANCH != 'master'
+  only_if: $CIRRUS_CRON == 'nightly'
   skip: "changesIncludeOnly('doc/**', '**.{md,rdoc,ronn,[1-8]}', '.document')"
   arm_container:
     # We use the arm64 images at https://github.com/ruby/ruby-ci-image/pkgs/container/ruby-ci-image .
@@ -74,6 +75,7 @@ task:
 yjit_task:
   name: Arm64 Graviton2 / $CC YJIT
   auto_cancellation: $CIRRUS_BRANCH != 'master'
+  only_if: $CIRRUS_CRON == 'nightly'
   skip: "changesIncludeOnly('doc/**', '**.{md,rdoc,ronn,[1-8]}', '.document')"
   arm_container:
     # We use the arm64 images at https://github.com/ruby/ruby-ci-image/pkgs/container/ruby-ci-image .