Allow manual invocation of verify TLS CI step (#3602)

The verify TLS CI step is flaky at the moment, so it would be helpful to
be able to run it manually against main to see if it was the PR that
broke it, or if it is its general flakiness.

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.github/workflows/ci-tls.yml

@@ -11,6 +11,7 @@ env:
 
 name: Verify client TLS configuration
 on:
+  workflow_dispatch:
   pull_request:
   push:
     branches: [main]