ci: allow manual dispatch

Gitea exposes no rerun endpoint in its API, so retrying a run after a
runner-side change otherwise requires pushing a commit that changes
nothing.

.gitea/workflows/ci.yml

@@ -24,6 +24,10 @@ name: CI
 on:
   push:
   pull_request:
+  # Lets a run be retried from the Actions page without inventing a commit.
+  # Gitea's API exposes no rerun endpoint, so without this the only way to
+  # re-test a runner-side change is to push something.
+  workflow_dispatch:
 
 jobs:
   backend: