workflows: CIFuzz: push -> workflow_dispatch

Otherwise this'll get too noisy quickly.

.github/workflows/ci-fuzz.yml

diff --git a/.github/workflows/ci-fuzz.yml b/.github/workflows/ci-fuzz.yml
index b2acf4d..fe8b5c7 100644
--- a/.github/workflows/ci-fuzz.yml
+++ b/.github/workflows/ci-fuzz.yml
@@ -1,5 +1,5 @@
 name: CIFuzz
-on: [pull_request, push]
+on: [pull_request, workflow_dispatch]
 jobs:
   Fuzzing:
     runs-on: ubuntu-latest