Add manual triggers to integration test workflow

These allow the maintainer to trigger a workflow run after a relevant external change.

Author: per1234

.github/workflows/test-integration.yml

@@ -23,10 +23,14 @@ on:
       - "action.yml"
       - "compilesketches/**"
 
+  repository_dispatch:
+
   schedule:
     # Run periodically to catch breakage caused by external changes.
     - cron: "0 12 * * THU"
 
+  workflow_dispatch:
+
 env:
   SKETCHES_REPORTS_PATH: sketches-reports
   TESTDATA_PLATFORMS_PATH: .github/workflows/testdata/platforms