change the trigger for running fuzzer tests

Since fuzzers benefit more from being run regularly on the openbsd tree,
and they take a long time, does it make more sense that the trigger is a
cron like the Coverity job? They sometimes don't work in branches anyway,
since the fuzz builds rely on external code being updated to match
the master branch.

.github/workflows/cifuzz.yml

@@ -1,5 +1,10 @@
 name: CIFuzz
-on: [pull_request]
+
+on:
+  workflow_dispatch:
+  schedule:
+  - cron: "0 0 * * *"
+
 jobs:
   Fuzzing:
     runs-on: ubuntu-latest