message: It is a bit cleaner to use os.LookupEnv than os.Getenv.
diff: @@ -24,8 +24,8 @@ import (
  )
  func tinkRootPath() (string, error) {
  - root := os.Getenv("TEST_SRCDIR")
  - if root == "" {
  + root, ok := os.LookupEnv("TEST_SRCDIR")
  + if !ok {
  return "", errors.New("TEST_SRCDIR not found")
  }
  path := root + "/google3/third_party/tink"
  