Simplify usage of os/exec

For now, keep the finishRunning() wrapper but use a straight cmd.Run()
call instead of the convoluted goroutine trying to catch signals.

It turns out that Unix process group handling is enough to interrupt
pending processes when stopping the run with something like a Ctrl+C
which should be enough.

Tested:
- Full e2e run with hack/e2e-test.sh, two tests failed but looks like
  they've been failing before this change.
- Started a hack/e2e.go -v -build and interrupted it with Ctrl+C,
  confirmed that build-release.sh was killed in the process.

hack/e2e.go

@@ -27,7 +27,6 @@ import (
 	"net/http"
 	"os"
 	"os/exec"
-	"os/signal"
 	"path"
 	"path/filepath"
 	"strconv"
@@ -63,7 +62,6 @@ const (
 )
 
 var (
-	signals = make(chan os.Signal, 100)
 	// Root directory of the specified cluster version, rather than of where
 	// this script is being run from.
 	versionRoot = *root
@@ -86,7 +84,6 @@ type ResultsByTest map[string]TestResult
 
 func main() {
 	flag.Parse()
-	signal.Notify(signals, os.Interrupt)
 
 	if *isup {
 		status := 1
@@ -295,40 +292,17 @@ func runBashUntil(stepName string, cmd *exec.Cmd) func() {
 	}
 }
 
-func finishRunningWithOutputs(stepName string, cmd *exec.Cmd) (bool, string, string) {
-	log.Printf("Running: %v", stepName)
-	stdout, stderr := bytes.NewBuffer(nil), bytes.NewBuffer(nil)
+func finishRunning(stepName string, cmd *exec.Cmd) bool {
 	if *verbose {
-		cmd.Stdout = io.MultiWriter(os.Stdout, stdout)
-		cmd.Stderr = io.MultiWriter(os.Stderr, stderr)
-	} else {
-		cmd.Stdout = stdout
-		cmd.Stderr = stderr
+		cmd.Stdout = os.Stdout
+		cmd.Stderr = os.Stderr
 	}
-
-	done := make(chan struct{})
-	defer close(done)
-	go func() {
-		for {
-			select {
-			case <-done:
-				return
-			case s := <-signals:
-				cmd.Process.Signal(s)
-			}
-		}
-	}()
-
+	log.Printf("Running: %v", stepName)
 	if err := cmd.Run(); err != nil {
 		log.Printf("Error running %v: %v", stepName, err)
-		return false, string(stdout.Bytes()), string(stderr.Bytes())
+		return false
 	}
-	return true, string(stdout.Bytes()), string(stderr.Bytes())
-}
-
-func finishRunning(stepName string, cmd *exec.Cmd) bool {
-	result, _, _ := finishRunningWithOutputs(stepName, cmd)
-	return result
+	return true
 }
 
 func printBashOutputs(headerprefix, lineprefix, output string, escape bool) {