ci: Disable migration compatibility tests for aarch64

Until 9.0 is out, we need to keep the aarch64 job disabled because the
tests always use the n-1 version of migration-test. That happens to be
broken for aarch64 in 8.2. Once 9.0 is out, it will become the n-1
version and it will bring the fixed tests.

We can revert this patch when 9.0 releases.

Signed-off-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240118164951.30350-4-farosas@suse.de
[peterx: use _SKIPPED rather than _OPTIONAL]
Signed-off-by: Peter Xu <peterx@redhat.com>

.gitlab-ci.d/buildtest.yml

@@ -217,10 +217,14 @@ build-previous-qemu:
     - QTEST_QEMU_BINARY_DST=./qemu-system-${TARGET}
           QTEST_QEMU_BINARY=../build/qemu-system-${TARGET} ./tests/qtest/migration-test
 
+# This job is disabled until we release 9.0. The existing
+# migration-test in 8.2 is broken on aarch64. The fix was already
+# commited, but it will only take effect once 9.0 is out.
 migration-compat-aarch64:
   extends: .migration-compat-common
   variables:
     TARGET: aarch64
+    QEMU_JOB_SKIPPED: 1
 
 migration-compat-x86_64:
   extends: .migration-compat-common