CI: run only tests against default and stable in scheduled jobs

We'd like to have nightly pipeline runs, to know if all goes well with changes
in Mercurial, but we don't want the jobs for Mercurial fixed versions to run.
9 jobs for topic/default/add-ci in 3 minutes and 28 seconds (queued for 18 seconds)
latest
Status Name Job ID Coverage
  Test
passed tests-4.6 #61694

00:01:03

passed tests-4.7 #61693

00:01:05

passed tests-4.8 #61692

00:01:06

passed tests-4.9 #61691

00:01:03

passed tests-5.0 #61690

00:01:07

passed tests-5.1 #61689

00:01:05

passed tests-5.2 #61688

00:01:05

passed tests-default #61687

00:01:15

passed tests-stable #61686

00:01:12