Allow the CI fuzz workflow to be manually triggered too

This adds workflow_dispatch as an event trigger for `cifuzz.yml`.

Since that should be sufficient to cover the rare case where one
wishes to run it outside of a pull request against the `main`
branch, this narrows the existing pull_request trigger to occur
only on `main`, as is already the case in the other workflows that
are intended to run more often than this one.

.github/workflows/cifuzz.yml

@@ -1,5 +1,8 @@
 name: CIFuzz
-on: [pull_request]
+on:
+  pull_request:
+    branches: [ main ]
+  workflow_dispatch:
 jobs:
  Fuzzing:
    runs-on: ubuntu-latest