Add courier to breakage test dispatch matrix (#116)

The courier repo's breakage workflow listens for
shared-docker-builders-updated but was never added to the dispatch
matrix, so its breakage test never triggers.

Closes ponylang/courier#13

Author: SeanTAllen

.github/workflows/rebuild-ponyc-based-images.yml

@@ -411,6 +411,7 @@ jobs:
           - ponylang/appdirs
           - ponylang/changelog-tool
           - ponylang/corral
+          - ponylang/courier
           - ponylang/crdt
           - ponylang/fork_join
           - ponylang/github_rest_api