run unittests in privileged docker container

The code coverage tests already run in a privileged container, so this
does not affect our security posture. Additionally, it will unblock
vsock unit tests in the vhost-user-vsock crate, see also
https://github.com/rust-vmm/vhost-device/pull/706 and
https://github.com/rust-vmm/vhost-device/pull/728

Signed-off-by: Patrick Roy <roypat@amazon.co.uk>

.buildkite/test_description.json

@@ -28,7 +28,10 @@
         "x86_64",
         "aarch64",
         "riscv64"
-      ]
+      ],
+      "docker_plugin": {
+        "privileged": true
+      }
     },
     {
       "test_name": "unittests-musl",
@@ -36,7 +39,10 @@
       "platform": [
         "x86_64",
         "aarch64"
-      ]
+      ],
+      "docker_plugin": {
+        "privileged": true
+      }
     },
     {
       "test_name": "clippy",