Fix rules for lektor check_new_strings job

They are currently written as if they were always all evaluated but the documentation states that they are in fact evaluated sequentially and the rule validates as soon as one of the condition is true. So right now this causes all projects to run this job when the commit is made on the default branch.

The fix is to use a boolean expression inside the first rule to validate both conditions are true at the same time.

https://docs.gitlab.com/ee/ci/jobs/job_control.html#cicd-variable-expressions