tweak GitHub Actions (#4037)

.github/workflows/ci.yml

@@ -1,5 +1,8 @@
 name: CI
-on: [ push, pull_request ]
+on:
+  pull_request:
+  push:
+    branches: [ master ]
 jobs:
   test:
     strategy:

.github/workflows/ufuzz.yml

@@ -2,7 +2,7 @@ name: Fuzzing
 on:
   pull_request:
   schedule:
-    - cron: "*/8 * * * *"
+    - cron: "0,8,15,23,30,38,45,53 * * * *"
 jobs:
   ufuzz:
     strategy: