terminate go-fuzz gracefully (w/ SIGINT) (#5973)
terminate go-fuzz gracefully (w/ SIGINT) (#5973)

and preserve exit code.

```
2021/01/26 03:34:49 workers: 2, corpus: 4 (8m28s ago), crashers: 0, restarts: 1/9976, execs: 11013732 (21596/sec), cover: 121, uptime: 8m30s
make: *** [fuzz-mempool] Terminated
Makefile:5: recipe for target 'fuzz-mempool' failed
Error: Process completed with exit code 124.
```

https://github.com/tendermint/tendermint/runs/1766661614

`continue-on-error` should make GH ignore any error codes.
tendermint/tendermint,tendermint/tendermint,tendermint/tendermint,tendermint/tendermint