Add emergency message for Mac worker outage (#1274)

diff --git a/buildkite/emergency.yml b/buildkite/emergency.yml
index 3207eea..64d4efd 100644
--- a/buildkite/emergency.yml
+++ b/buildkite/emergency.yml
@@ -18,5 +18,5 @@
 # Instead, simply replace all values with empty strings ("").
 ---
 message: ""
-issue_url: ""
+issue_url: "MacOS machines are offline due to network maintenance. We expect the outage to continue until 6pm EDT today."
 last_good_bazel: ""