fix: ignore exit codes when child killed by manager (#6767)
### Description
Fixes https://github.com/vercel/turbo/issues/6754
As of `pnpm@8.10.0` `pnpm run script` will respond to a `SIGINT` with an
exit code of `0` (See https://github.com/pnpm/npm-lifecycle/pull/41).
`turbo` would see this exit code and assume the task successfully
completed which results in unwanted behavior on our part.
In Go we don't run into this issue as we [lock the stop channel during
shutdown](https://github.com/vercel/turbo/blob/main/cli/internal/process/child.go#L205)
and then [will fail this check once the lock is
released](https://github.com/vercel/turbo/blob/main/cli/internal/process/child.go#L251)
meaning that we would never send the underlying exit code to the exit
channel.
Huge thanks to @laat for reporting the bug with a paired down
reproduction repo and for finding the underlying PR that changed
`pnpm`'s behavior.
### Testing Instructions
Updating existing unit tests to check that we get a `ChildExit::Killed`
instead of the underlying process exit code.
Closes TURBO-1893
Co-authored-by: Chris Olszewski <Chris Olszewski>
