Fix breakage test event type mismatch (#81)

The dispatch type was shared-docker-linux-builders-updated but the
sender (shared-docker) dispatches shared-docker-builders-updated,
so the breakage test never fired.

Closes #71

Author: SeanTAllen

.github/workflows/breakage-against-ponyc-latest.yml

@@ -2,7 +2,7 @@ name: Test against ponyc nightly
 
 on:
   repository_dispatch:
-    types: [shared-docker-linux-builders-updated]
+    types: [shared-docker-builders-updated]
 
 permissions:
   packages: read