Give the CI fuzzer more time

Currently the CI fuzzer isn't on the critical path to completion.
So we can extend the amount of fuzzing time without impacting the
critical path.

.github/workflows/cifuzz.yml

@@ -15,7 +15,7 @@ jobs:
      with:
        oss-fuzz-project-name: 'gitoxide'
        language: rust
-       fuzz-seconds: 600
+       fuzz-seconds: 840
    - name: Upload Crash
      uses: actions/upload-artifact@v3
      if: failure() && steps.build.outcome == 'success'