travis: Disable arm64 failures

Travis is recently failing a lot for ARM builds with issues like:

* #10945
* #10977

Allow failures for the moment while we triage these so that all PRs are
not affected by these known issues.

CC: Jianlin Lv <Jianlin.Lv@arm.com>
Signed-off-by: Joe Stringer <joe@cilium.io>

.travis.yml

@@ -12,6 +12,10 @@ arch:
   - amd64
   - arm64
 
+jobs:
+  allow_failures:
+  - arch: arm64
+
 if: branch = master OR type = pull_request
 
 addons: