Stop builder instance in integration using Ctlr-C signals

For the Rbuilder instance running with the integration tests, we should stop the process using `Ctlr-C` signals instead of [killing the process](https://github.com/flashbots/rbuilder/blob/777e822c0366e79621bb6f26d15a073f30f960c1/crates/rbuilder/src/integration/playground.rs#L172) to ensure that `Ctrl-C` works as expected.

It could catch errors like #349