This test is broken, as it first sets environment variables, and then
immediately unsets it again.
This only worked by chance with the legacy tracer, and breaks the Lua
tracer.
Refactor the PR checks workflow into two workflows:
PR checks and Runner checks.
This does not change the actual check jobs that are run.
It also does not change the expected check names
(which rely only on the job name, not the workflow name).
This makes it easier to inspect workflow run summaries in the UI
and to separately retry subsets of failed jobs in case of flakiness.
In future we will clean up the Runner checks,
since this is a deprecated component.
