blob/gcsblob: anonymous bucket access follow up to #3302

[calebbrown]

From #3302:

I finally got around to testing #3306. Unfortunately it doesn't work.

I suspect the issue is the creds used when there is no ADC is empty:

creds = &google.Credentials{}

So creds.TokenSource is not set and the API requests fail.

One possibility for solving this is using something like the following to generate a valid creds instance:

creds, _ = google.CredentialsFromJSON(ctx, []byte(`{"type": "service_account"}`))

Or use golang.org/x/oauth2/jwt to create the token source:

creds.TokenSource = &jwt.Config{
		Scopes:       []string{"https://www.googleapis.com/auth/cloud-platform"},
		TokenURL:     google.JWTTokenURL,
	}.TokenSource(ctx)

[calebbrown]

#3317 fixes this issue.