cifuzz: fuzz the master branch on push

Apart from running CIFuzz for each relevant PR, let's run it
unconditionally for each push to master to detect possible issues
(caused by ignored PRs, etc.).

Followup to 94f660a8fe.

.github/workflows/main.yml

@@ -12,6 +12,9 @@ on:
       - 'src/**'
       - 'test/fuzz/**'
       - 'tools/oss-fuzz.sh'
+  push:
+    branches:
+      - master
 jobs:
  Fuzzing:
    runs-on: ubuntu-latest