cfl: increase a prune timeout

```
  File "/usr/local/lib/python3.8/dist-packages/clusterfuzz/_internal/bot/fuzzers/libFuzzer/engine.py", line 507, in minimize_corpus
    raise TimeoutError('Merging new testcases timed out\n' + result.output)
TimeoutError: Merging new testcases timed out
```

The prune duration is come from `FUZZ_SECONDS` // `fuzzers_left_to_run`.
The patch increase a prune timeout, it seems due to a huge corpus
and a number of fuzzers pruning takes more time and time is not enough.

See also google/clusterfuzzlite#89

Author: ligurio

.github/workflows/cflite_cron.yml

@@ -58,7 +58,7 @@ jobs:
         id: run
         uses: google/clusterfuzzlite/actions/run_fuzzers@main
         with:
-          fuzz-seconds: 1800
+          fuzz-seconds: 3600
           github-token: ${{ secrets.GITHUB_TOKEN }}
           minimize-crashes: true
           mode: 'prune'