message/pipeline: skip test if the 'go' executable is not found

Fixes golang/go#32844

Change-Id: I9d5364df4f827db4de3c144a565c1d05d8a181bd
Reviewed-on: https://go-review.googlesource.com/c/text/+/192325
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>

message/pipeline/pipeline_test.go

@@ -12,6 +12,7 @@ import (
 	"fmt"
 	"io/ioutil"
 	"os"
+	"os/exec"
 	"path"
 	"path/filepath"
 	"runtime"
@@ -30,6 +31,10 @@ func TestFullCycle(t *testing.T) {
 	if runtime.GOOS == "android" {
 		t.Skip("cannot load outside packages on android")
 	}
+	if _, err := exec.LookPath("go"); err != nil {
+		t.Skipf("skipping because 'go' command is unavailable: %v", err)
+	}
+
 	const path = "./testdata"
 	dirs, err := ioutil.ReadDir(path)
 	if err != nil {