I think GitHub action checks do not show up in PRs in jdk21u repo.

See e.g. PR:
 https://github.com/openjdk/jdk21u/pull/16

...and associated run:
 https://github.com/shipilev/jdk21u/actions/runs/5668436021

Should it be explicitly enabled, like we did in SKARA-1132, for example?

Note: Rob McKenna (current 21u maintainer) is added to Watchers.