Limit cifuzz.yml to the `varnishcache` github org

Limit the fuzzer runs to PRs on `varnishcache`'s repositories. The build
scripts won't work correctly from anywhere else anyways.

.github/workflows/cifuzz.yml

@@ -2,6 +2,7 @@ name: CIFuzz
 on: [pull_request]
 jobs:
   Fuzzing:
+    if: github.repository_owner == 'varnishcache'
     runs-on: ubuntu-latest
     steps:
     - name: Build Fuzzers