Bug#1071769: golang-github-google-cel-go: FTBFS: panic: interface conversion: ref.Val is *types.Err, not types.Bool

[Santiago Vila]

retitle 1071769 golang-github-google-cel-go: FTBFS: FAIL: 
TestTimestampGetDayOfMonth
thanks

I'm putting the test which failed in the subject to avoid confusion.

Thanks.