gitlab-ci: Disable the riscv64-debian-cross-container by default

This job is failing since weeks. Let's mark it as manual until
it gets fixed.

Message-Id: <82aa015a-ca94-49ce-beec-679cc175b726@redhat.com>
Acked-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Thomas Huth <thuth@redhat.com>

.gitlab-ci.d/container-cross.yml

@@ -95,6 +95,7 @@ riscv64-debian-cross-container:
   allow_failure: true
   variables:
     NAME: debian-riscv64-cross
+    QEMU_JOB_OPTIONAL: 1
 
 # we can however build TCG tests using a non-sid base
 riscv64-debian-test-cross-container: