ci: allow manual dispatch of the cross-platform E2E workflow

GitHub stopped delivering push/pull_request events for this branch;
workflow_dispatch provides a manual escape hatch (CI and build-and-make
already have one).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.github/workflows/e2e-tests.yaml

@@ -7,6 +7,7 @@ on:
     pull_request:
         branches:
             - master
+    workflow_dispatch:
 
 jobs:
     electron-e2e-tests: