sweet: ignore ErrProcessDone from SIGKILL to cockroachdb instances

Sending SIGKILL is racy with the program exiting normally via SIGTERM.
If this happens, we might see ErrProcessDone, which is safe to ignore.
Seen at least once on the builders.

Change-Id: Iab8190e46e4d9f00c6ef059909feca92aaaa798e
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/615135
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

sweet/benchmarks/cockroachdb/main.go

@@ -301,7 +301,9 @@ func (i *cockroachdbInstance) shutdown() (killed bool, err error) {
 	case <-time.After(1 * time.Minute):
 		// If it takes a full minute to shut down, just kill the instance
 		// and report that we did it. We *probably* won't need it again.
-		if err := i.cmd.Process.Signal(syscall.SIGKILL); err != nil {
+		// We're also racing here with the process exiting, so we could see
+		// ErrProcessDone; ignore it if we do.
+		if err := i.cmd.Process.Signal(syscall.SIGKILL); err != nil && !errors.Is(err, os.ErrProcessDone) {
 			return false, fmt.Errorf("failed to send SIGKILL to instance %s: %v", i.name, err)
 		}
 		killed = true