Change default shell signal from SIGTERM to SIGINT

For historical reasons when we send interrupts to process groups, we default to using `SIGTERM`:

https://github.com/buildkite/agent/blob/b9bc5ecc8e67fd6734d8930fb5ebb848902d99f2/process/signal.go#L35-L38

When we send `SIGTERM` to a process group, bash exits without waiting on its child processes to exit safely. In many cases, the child processes end from other effects, such as `SIGPIPE` caused by trying to read or write over an orphaned pipeline. This unreliability can lead to lost output or incorrect shutdowns of subprocesses. A script author could potentially work around this using a signal handler. A better approach would be to send a `SIGINT` signal instead, which will be followed by a `SIGKILL` if the group takes too long to respond. `SIGINT` only gets propagated to foreground child processes, so special case handling will still be required for users who are backgrounding processes.

Most processes treat `SIGINT` the same as `SIGTERM`, but we can't be sure if anyone is relying on this behaviour. Therefore, this should be considered a breaking change. A user can experiment with this change ahead of time by using the configurable cancel signals introduced by #1041 and #1390.

I've tried to carefully research this and get the above details correct, but please feel free to contribute any corrections or details I might have missed.