testenv: look for "go" executable in path.

--- src/go/build/deps_test.go
+++ src/go/build/deps_test.go
@@ -168,7 +168,7 @@ var pkgDeps = map[string][]string{
 	"testing":          {"L2", "flag", "fmt", "os", "runtime/debug", "runtime/pprof", "runtime/trace", "time"},
 	"testing/iotest":   {"L2", "log"},
 	"testing/quick":    {"L2", "flag", "fmt", "reflect"},
-	"internal/testenv": {"L2", "os", "testing"},
+	"internal/testenv": {"L2", "os", "os/exec", "testing"},
 
 	// 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.
--- src/internal/testenv/testenv.go
+++ src/internal/testenv/testenv.go
@@ -12,6 +12,7 @@ package testenv
 
 import (
 	"os"
+	"os/exec"
 	"runtime"
 	"strings"
 	"testing"
@@ -36,6 +37,9 @@ func HasGoBuild() bool {
 			return false
 		}
 	}
+	if _, err := exec.LookPath("go"); err != nil {
+		return false
+	}
 	return true
 }