code: func AsDefaultCredentials(ctx *AssertContext) {
  	conf := ctx.Config
  	Expect(conf).ToNot(BeNil(),
  		"cannot set static AssertContext without config")
  
  	tempFile, err := ioutil.TempFile("", "bosh-gcscli-service-account-file")
  	Expect(err).ToNot(HaveOccurred())
  	defer tempFile.Close()
  
  	tempFile.WriteString(ctx.serviceAccountFile)
  
  	ctx.serviceAccountPath = tempFile.Name()
  	os.Setenv(GoogleAppCredentialsEnv, ctx.serviceAccountPath)
  
  	conf.CredentialsSource = config.DefaultCredentialsSource
  }
docs: // AsDefaultCredentials configures the AssertContext to authenticate using
  // Application Default Credentials populated using the
  // testing service account file.