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
1 job for !13 with fix-rules in 1 minute and 17 seconds (queued for 4 seconds)
latest detached
Status Name Job ID Coverage
  Test
passed test #53298

00:01:17