testenv: look for "go" executable in path.

--- src/go/build/deps_test.go
+++ src/go/build/deps_test.go
@@ -182,17 +182,17 @@ var pkgDeps = map[string][]string{
 	"runtime/debug":  {"L2", "fmt", "io/ioutil", "os", "time"},
 	"runtime/pprof":  {"L2", "compress/gzip", "context", "encoding/binary", "fmt", "io/ioutil", "os", "text/tabwriter", "time"},
 	"runtime/trace":  {"L0"},
 	"text/tabwriter": {"L2"},
 
 	"testing":          {"L2", "flag", "fmt", "internal/race", "os", "runtime/debug", "runtime/pprof", "runtime/trace", "time"},
 	"testing/iotest":   {"L2", "log"},
 	"testing/quick":    {"L2", "flag", "fmt", "reflect", "time"},
-	"internal/testenv": {"L2", "OS", "flag", "testing", "syscall"},
+	"internal/testenv": {"L2", "OS", "os/exec", "flag", "testing", "syscall"},
 
 	// L4 is defined as L3+fmt+log+time, because in general once
 	// you're using L3 packages, use of fmt, log, or time is not a big deal.
 	"L4": {
 		"L3",
 		"fmt",
 		"log",
 		"time",
--- src/internal/testenv/testenv.go
+++ src/internal/testenv/testenv.go
@@ -43,16 +43,19 @@ func HasGoBuild() bool {
 	switch runtime.GOOS {
 	case "android", "nacl":
 		return false
 	case "darwin":
 		if strings.HasPrefix(runtime.GOARCH, "arm") {
 			return false
 		}
 	}
+	if _, err := exec.LookPath("go"); err != nil {
+		return false
+	}
 	return true
 }
 
 // MustHaveGoBuild checks that the current system can build programs with ``go build''
 // and then run them with os.StartProcess or exec.Command.
 // If not, MustHaveGoBuild calls t.Skip with an explanation.
 func MustHaveGoBuild(t testing.TB) {
 	if os.Getenv("GO_GCFLAGS") != "" {