Pipeline: turn off the arm64 pipeline for non-DDs

The default arm64 (and riscv64) runners on Salsa are only available for
DDs. Instead of enabling the runners for the main namespace, they are
only turned off for non-DDs.

See the documentation at
https://salsa.debian.org/salsa-ci-team/pipeline#enable-building-on-arm-and-risc-v

debian/gitlab-ci.yml

@@ -47,6 +47,8 @@ ISO amd64:
 ISO arm64:
   extends:
     - .lb-build
+  rules:
+    - if: $CI_PROJECT_ROOT_NAMESPACE != "rclobus-guest"
   variables:
     BUILD_ARCH: 'arm64'
   tags: